THE MANTLE AND SHELL OF SOLEMYA PARKINSONI (PROTOBRANCHIA: BIVALVIA)

The shell of Solemya exhibits considerable flexibility which is further enhanced by the marked extension of the periostracum beyond the calcareous portions of the valves. This fcature, more than any other, has made possible the habit, unique among bivalves, of burrowing deep within the substrate without direct contact with the water above. The inner calcareous layer of tho valves is restricted to a small area near the umbones while the outer calcareous layer is thin and contains a high proportion of organic material. The shell conchiolin consists mainly of protein, varying in composition, but much of it strengthcned by quinone-tanning, and in ccrtain regions probably by the presence of appreciable quantities of chitin. The ligament, although superficially resembling an amphidetic structure, is opisthodetic, the extcnsion anterior to the umbones consisting of anterior outer layer only.
The mantle is characterized by an extension of the outer fold of the mantle margin which has effected equally both the inner and outer surfaces of this fold. The secretory epithelium and the modified pallial musculature, contraction of which results in the intucking and plaiting of the periostracum, is dcscribed. Simple tubular oil glands open at the mantlo margin and are responsible for the water-repellent nature of the periostracum.
The form of the mantlelshell and that of the enclosed body are discussed and compared with those of other bivalves in which elongation of the mantle/shell is achieved in a different way. It is concluded that the mantlelshell of Solemya is of little value in determining its relationships, and that the greatly elongatod ligament, the edentulous hinge and the flexible shell are all adaptations to a specialized mode of life.     

Further twists in gastropod shell evolution

The manner in which a gastropod shell coils has long intrigued laypersons and scientists alike. In evolutionary biology, gastropod shells are among the best-studied palaeontological and neontological objects. A gastropod shell generally exhibits logarithmic spiral growth, right-handedness and coils tightly around a single axis. Atypical shell-coiling patterns (e.g. sinistroid growth, uncoiled whorls and multiple coiling axes), however, continue to be uncovered in nature. Here, we report another coiling strategy that is not only puzzling from an evolutionary perspective, but also hitherto unknown among shelled gastropods. The terrestrial gastropod Opisthostoma vermiculum sp. nov. generates a shell with: (i) four discernable coiling axes, (ii) body whorls that thrice detach and twice reattach to preceding whorls without any reference support, and (iii) detached whorls that coil around three secondary axes in addition to their primary teleoconch axis. As the coiling strategies of individuals were found to be generally consistent throughout, this species appears to possess an unorthodox but rigorously defined set of developmental instructions. Although the evolutionary origins of O. vermiculum and its shell's functional significance can be elucidated only once fossil intermediates and live individuals are found, its bewildering morphology suggests that we still lack an understanding of relationships between form and function in certain taxonomic groups.     

Shell biomechanics in the bivalve Laternula

The ligament in Laternula is modified into a complex mechanical structure composed of rigid and flexible parts with multiple connection points. Rather than allowing the valves to move with respect to each other, this ligament tends to keep them immobile. Providing space inside the shell for the modified ligament requires a secondary increase in shell curvature of the umbones. This is achieved by the shell flexing by muscular contraction and stiffening while flexed through the construction of a rigid buttress. A vertical slit in each valve, produced by secondary resorption and breakage of shell material, confines mechanical stress to the posterior slope of the shell. □ Mollusca, Bivalvia, Laternulidae, Holocene, Indo-Pacific, functional morphology.     

Biology and functional morphology of a new species of endolithic Bryopa (Bivalvia: Anomalodesmata: Clavagelloidea) from Japan and a comparison with fossil species of Stirpulina and other Clavagellidae

Abstract. A new species of Clavagellidae, Bryopa aligamenta, from Okinawa, Japan, is described. The species is endolithic in living corals, with the left valve cemented to the crypt wall, as in all clavagellids. The free right valve exhibits an unusual growth pattern, with commarginal lines seemingly arising from the posterior valve margin and extending towards the anterior. This results from: (i) progressive anterior erosion of the umbones, probably as a consequence of the boring process; (ii) the apparent migration posteriorly, as the umbones are eroded, of the dorso‐ventral growth axis of the shell; and (iii) enhanced posterior inter‐commarginal growth. Unlike other clavagellid genera and species, however, there is no discernible primary ligament, at least in the adult. It is possible, however, that if a juvenile ligament were present (as in B. lata), it too would be lost as a consequence of antero‐dorsal erosion during boring. To retain valve alignment in the absence of a primary ligament, and possibly upon reaching an adult size, the mantle lays down alternating layers of calcium carbonate and proteinaceous periostracum onto the interior surface of the shell to thicken it, most noticeably marginally and, especially, posteriorly. The two valves are united dorsally, therefore, by thin layers of periostracum that probably exert a minimal opening force. B. aligamenta is, however, further characterised by large adductor, pallial, and siphonal retractor muscles so that the entire animal is encased tightly within an internally strengthened shell within a crypt. Movement must be minimal, blood being pumped into pallial haemocoels to push open the valves and extend the siphons. Despite a suggestion to the contrary, Bryopa is retained in the Clavagellidae, its unusual growth processes resulting from an endolithic life style within living corals. The fossil clavagellid Stirpulina bacillus, from the Pliocene/Pleistocene of Palermo, Sicily, Italy, was, unlike Bryopa aligamenta and other clavagellids, endobenthic, with a long adventitious tube and anterior watering pot superficially similar to species of Penicillidae, another family of the Clavagelloidea. Furthermore, as in all clavagellids only the left valve is fused into the fabric of the tube, the right being free within it. In all penicillids, both valves are fused into the fabric of their tubes. The watering pots of the fossil S. coronata, S. vicentina, and S. bacillus, moreover, are formed in a different manner to that of penicillids, by progressive encasement of the right valve inside the tube. In penicillids, the tube is secreted in a single event from the general mantle surface and the incorporation of both valves into its fabric. The constituent genera of the Clavagellidae thus constitute an example of parallel evolution with members of the Penicillidae.     

WACHSTUMS-ÄNDERUNGEN IN DER ONTO-GENESE PALÄOZOISCHER AMMONOIDEEN

Ontogenetic allometries (heterauxesis according to Huxley) of about 50 Carboniferous goniatite species from Germany, Spain and USA have been studied by means of geometrical measurements of shell cross sections. They indicate changes of proportions during ontogeny between coiling spiral, umbilical spiral, and whorl width. The allometry constants change suddenly at defined stages. Thus the ontogenetic growth pattern is polyphase, each phase being determined by a different allometric formula.
Ontogenetische Allometrien wurden bei etwa 50 karbonischen Goniatiten-Arten aus Deutschland, Spanien und USA an Hand von Gehäuse-Querschnitten untersucht. Während der Ontogenese erfolgen mehrfach Proportionsverschie-bungen der äußeren Spirale, der Nabelspirale und der Windungsbreite, die sich durch scharfen Wechsel von Allometrie-Konstanten an definierten Stellen aus-drücken. Das Gehäusewachstum erfolgt somit in mehreren Phasen, die von unterschiedlichen Allometrieformeln bestimmt sind.     

Regulation of spiral coiling in the terrestrial gastropod Sphincterochila: An experimental test of the road-holding model

Hutchinson's ('89) road-holding model states that spiral ornaments of the snail shell (keels and low-curvature areas) dictate the growth path of the subsequent whorl, which in turn gives the signal for attachment of the next whorl. Experiments were performed with two species of the terrestrial snail Sphincterochila in order to test the role of the external keel in determining the correct coiling of successive turns. Experiments substituted a ridge made of silicone for the keel. This ridge ran either (1) abapical or (2) adapical of the original keel. In mode (1), subsequent growth continued by taking the false keel as the adapical limit of the whorl. In only very few instances of mode (2) did the whorls extend incipiently slightly adapical of the path of the original keel. Our results confirm that the keel is an important reference for the coiling strategy of the snail, although the keel itself probably does not constitute the reference, but rather the two flat ramps into which the keel divides the outer lip of the aperture. J. Morphol. 235:249–257, 1998. © 1998 Wiley-Liss, Inc.     

The tube-like structures on the juvenile shells of earliest strophomenides and billingsellides as evidence of their life cycles

Possible life cycle of some ancient plectambonitoids (order Strophomenida) from the Middle Ordovician of Russia is reconstructed based on the well-preserved specimens composing the ontogenetic series. Four regions may be distinguished on their shell surface: protegulum, brephic shell, neanic shell and adult shell. The posterior margin of ventral protegulum bears pedicle sheath, which is a tubular outgrowth with a 40-μm-wide aperture at the distal end. The protegulum and brephic shell have common type of microstructure that possibly is spherular; the neanic and adult shells are fibrous. The strophomenide ontogeny possibly was similar to that of recent discinides. The strophomenide life cycle possibly included the planktotrophic juvenile stage; the protegulum and brephic shell were formed in the water column. The aperture of the pedicle sheath was possibly used as an anal opening of the floating juvenile and as an attachment organ during the settlement; at early adult stages, the sheath erased, the anus closed, and the animal started to lie on the ventral valve. The origin of the order Strophomenida and its relative groups is possibly connected with the loss of the pedicle lobe; judging by some strophomenide morphological features, true pedicle was present in the strophomenide ancestors. The tubes on the ventral umbones of strophomenides and billingsellides are not homologous as pedicle sheaths of strophomenides are formed at the planktotrophic swimming stage, and the tubes surrounding the pedicles of billingsellides were formed by deltidial plates of almost adult shell after settling.     

COMPARATIVE STUDY ON EARLY ONTOGENY OF COMPOUND LEAVES IN LARDIZABALACEAE

The early ontogeny of the pinnately, palmately, and ternately compound leaves in the Lardizabalaceae was studied by SEM. The leaf primordium of each of the three leaf types emerges as an identical short protrusion on the shoot apex; the leaf primordium produces the first leaflet initials laterally on its margin. Successive acropetal growth of the leaf axis and the following inception of the leaflet primordia are responsible for the pinnately compound leaf, whereas short basipetal growth accompanied with initiation of two or more pairs of leaflet initials results in a palmately compound leaf. If no elongation of the leaf axis nor additional inception of leaflet primordia occur during early ontogeny, a ternate leaf ensues.     

A DEVELOPMENTAL CONSTRAINT IN CERION,WITH COMMENTS ON THE DEFINITION AND INTERPRETATION OF CONSTRAINT IN EVOLUTION

Since orthodox evolutionary theory is functionalist, constraints attain their most important positive meaning as channels of change imposed by historical and formal determinants, rather than by immediate natural selection. Since ontogeny is the usual locus of expression for these determinants, developmental constraint is an appropriate, general term. A particular developmental constraint in Cerion, most variable of West Indian land snails, stands out for two reasons: 1) it is simply and inexorably defined as a consequence both of formal principles (coiling of tube about axis) and of historical contingencies in Cerion's invariant allometry of growth; 2) it is pervasive in its influence, underlying major patterns of variation in every Cerion study I have ever undertaken. I refer to this pattern as the “jigsaw constraint.” When whorls are large and final size is limited, adult shells must grow fewer whorls. In Cerion, this obvious fact is promoted from trivial to important because complex allometries impose a substantial set of further consequences for form upon this basic trade-off of whorl size and whorl number. I show that this complex of consequences dominates patterns of natural variation in Cerion at all levels (among shells within samples, between samples in the geographic variation of single species, and between species in multitaxon faunas). It also sets patterns of hybridization between taxa. This paper is primarily a compendium of such examples. It is designed to illustrate the importance of this constraint by the fundamental criterion of relative frequency.     

Significance of epidermal fusion and intercalary growth for angiosperm evolution

The ancestral angiosperm flower probably had many separate elements in each floral whorl (sepals, petals, stamens and carpels). Derived character states include "fusion" of elements within a whorl (cohesion) and fusion between whorls (adhesion), as well as epigyny and the emergence of the other floral elements from the apex of the fused carpels. This article considers the roles of epidermal fusion and intercalary growth in the phylogeny and ontogeny of fused floral elements, and the importance of fusion for angiosperm evolution.     

Segmental growth in planulate ammonites: inferences on costal function

Checa, Antonio & Westermann, Gerd E. G. 1989 01 15: Segmental growth in planulate ammonites: inferences on costal function. Lethaia , Vol. 22, pp. 95–100. Oslo. ISSN 0024–1164.
In planulate Ammonitina, the directions of costae and their parts coincide with the growth lines, both being accreted in parallel and subtriangular increments, i.e. segments. This implies that the growing margin was consistently corrugated (plicated) parallel to the edge of the peristome. The adventral separation of the ribs caused by the coiling increased with whorl involution and was compensated for by costal division into primaries and secondaries. We propose that costation of the shallow-water, nekto-benthic plandates reduced the stiffness and increased effective thickness of their growing peristome. Ribs functioned (primarily or secondarily) as a protection for the immature, fragile aperture against predators and impacts.□ Ammonite shell, Jurassic, morphology, growth lines, ornamentation, costae .     

Mechanical constraints on aperture form in gastropods

Gastropod apertures reflect expanded states of their mantle edge under variable boundary conditions. The apertures are divided into two groups: apertures without distinct overlap zones (e.g., whorl overlap) and those with overlap zones. Each group follows a unique morphological rule. Apertures without overlap zones are generally circular in outline. Apertures with overlap zones are either elongated perpendicularly to the overlap zone or inflated abapically. Moreover, the latter abapically inflated apertures are generally accompanied by a straight section anterior to the overlap zone along the columellar axis (columellar part). Numerical analysis of an elastic double membrane tube whose main frame simulates the gastropod mantle indicates that these morphological rules are the products of mantle edge expansion under the condition that the head-foot mass presses against the mantle edge in both the overlap zone and the columellar part. The mantle edge in these two zones is thus in a completely or partly fixed boundary condition at the moment of shell growth. The rest of the mantle edge is free to expand either symmetrically or asymmetrically. It is hypothesized that the head-foot mass is a driving force for regulating the pattern of shell coiling and apertural shape.     

Covariation of morphological characters in the Triassic ammonoid Czekanowskites rieberi

The Triassic ammonoid Czekanowskites rieberi displays a covariation of morphological charac ters, which is rather common in ammonoids. Its morphological spectrum ranges from laterally compressed, involute, weakly ribbed forms to depressed, semiinvolute, strongly ribbed forms. In order to study this covariation, fifteen axially cut specimens have been analyzed by means of image analysis, which allows us to obtain the ontogenetic record of radii, area and perimeter of the individual whorl cross-sections. A logarithmic model of growth has been applied. Our data indicate that, owing to the covariation, the radii from the origin to the venter and to the umbil ical seam of a given whorl section vary inversely in order to maintain the relative position of the center of gravity of the whorl cross-section both throughout the ontogeny of single specimens and within the population. This influences hydrostatic parameters, such as the position of the center of mass and the orientation and stability of the shell. Since the ontogenetic record of the angular length of the body chamber is not known, we have calculated those hydrostatic varia bles using two mutually exclusive assumptions: (1) the angular length of the body chamber was constant throughout ontogeny and (2) the volume of the body chamber grew monotonically with the revolution angle. Fluctuations of the three hydrostatic variables were always less important in the first assumption. In any case, the spectrum of, for example, theoretical orien tations is comparable to those observed in the species of present-day Nautilus. The range of adult body-chamber length observed in C. rieberi is much narrower than the theoretical adult body-chamber length calculated under the second assumption which indicates that a certain control over this parameter existed in the natural population, probably in order to maintain a narrow range in orientation and stability. The excess or deficit in soft-body weight was probably compensated by inverse variations in shell-wall weight. The main conclusion is that, despite the extreme morphological variability, hydrostatic and, possibly, hydrodynamic properties of the population remained within narrow limits.     

Sectorial expansion and shell morphogenesis in molluscs

Checa, A. 1991 01 15: Sectorial expansion and shell morphogenesis in molluses. Lethaia . Vol. 24, pp. 97–114. Oslo. ISSN 0024–1164.
Any coiled shell can be described as a series of independent helicospirals that join homologous points along the shell surface. The cross-section is therefore seen as a set of points. obtained at its intersection with the helicospirals. Any cross-sectional sector contained between two adjacent points is capable of expanding or contracting during the development and a differential parameter ( L'/L ) has been devised to quantify this expansion rate. The morphometrics so obtained is here called sectorial expansion analysis. This analysis and other related procedures have been applied on cross-sections and apertures in selected Molluxa with the aim of evaluating the incidence of sectorial expansions on shell shape. Those parameters affecting whorl expansion rate and whorl overlapping may be directly modified by sectorial expansions. Changes in the mode of coiling (curvature and torsion) often, but not always, involve sectorial expansions, perhaps as a constructional feature. This approach reveals the advantages and drawbacks of the present analysis as compared to previous theoretical models. Sectorial expansion. morphogenesis. ornamentation, septal suture, coiled shell, gastropods, bivalues. ammonites .     

THE CONSEQUENCES OF BEING DIFFERENT: SINISTRAL COILING IN CERION

The overwhelming predominance of dextral coiling in gastropods is an outstanding and puzzling phenomenon. A few sinistral specimens (left coiling individuals) have been found in many dextral species. Only six sinistral shells have ever been found in Cerion; we base this analysis on the five available shells. We ask whether reversed symmetry is a simple either-or switch without further consequences for shell form, or whether sinistrality engenders associated effects, making left-coiling shells unlike their dextral deme-mates in other ways. All five sinistral shells differ in features of size and coiling late in growth, leading to relatively small apertures and a slight twist in the axis of coiling. We detect and measure this effect as follows: in multivariate morphospace, sinistrals occupy peripheral positions among their dextral deme-mates; in univariate analysis, sinistrals are consistently different for a set of characters involving covariance patterns never before seen in a decade of studies on ontogenetic and age-standardized variation in dextrals; a bootstrap procedure does not recover similar patterns in randomly constituted samples of dextrals matching the true sinistral distribution; direct x-ray measures of the coiling axis detect its slight twist in sinistrals. We discuss the implications of these unsuspected associations for the issues of developmental constraint upon the evolution of morphology.     

Regulation of spiral growth in planorbid gastropods

Extensive experimentation has been performed on the planorbid Planorbarius metidjensis in order to determine which mechanism allows the snail to coil its shell regularly. Individuals of this species, like all Planorbidae, are permanently active and secrete their shells while crawling on the substrate. Experiments consisted of attaching weights to either side of the shell (which is carried almost vertically) in an umbilical position; these weights cause the shell to fall towards the substrate on the loaded side. It can be demonstrated, qualitatively and quantitatively, that during further growth the shell tube deviated initially (i.e. within the first half whorl after loading) towards the loaded side. In a later stage, when the animal is able to re-balance the shell-load complex by muscular activity, the shell tube gradually deviates away from the loaded side. This behaviour is to be expected if, after loading, secretion of the shell continued with the aperture parallel to the substrate and forming a constant angle with the direction of growth. The main implication is that in normal conditions the living posture largely controls the correct coiling of the shell. Minor experiments made with another planorbid species, Gyraulus laevis , confirm these conclusions. The growth pattern of planorbids requires that the snail has constant information on the orientation of the shell with respect to the substrate. This is permitted by the particular physiological ecology of this group, members of which, unlike terrestrial gastropods, are permanently active.     

HYDRAULIC BURROWING IN THE BIVALVE MYA ARENARIA LINNAEUS (MYOIDEA) AND ASSOCIATED LIGAMENTAL ADAPTATIONS

The burrowing behaviour of the bivalve Mya arenaria from tidalflats of the Dutch Wadden Sea has been observed and recorded.Compared to other bivalves, M. arenaria is a very slow burrower,its burrowing behaviour being unique among bivalves since itis based essentially on the ejection of water through the pedalgape, with little assistance by the foot, which performs onlyan anchoring function. Water ejection is specially powerfuland individual jets may last several seconds, thus constitutingan effective means of removing sand from below the animal duringdigging. This hydraulic burrowing is more effective in loosesandy than in cohesive muddy substrates. Water ejection is providedby the ability of the bivalve to rock its valves across a dorsoventralaxis. This rocking motion implies special modifications of theligamental area. The ligament is conical in appearance and runsdorso-ventrally between the two chondrophores which are placedin two planes parallel to the cardinal axis. During rockingthe whole ligament acts in torsion and the lamellar layer ofthe ligament opposes closing of the anterior part of the valves.During normal adduction of the valves the ligament acts in bending,the axis of motion being placed internally with respect to thecardinal axis. This leads to approaching of the umbones withcomplete adduction and to resorption of the left umbo. Fromthe adaptive point of view, the slow hydraulic mode of burrowingis sufficient to cope with the slow sedimentation and erosionrates of the tidal flats in which M. arenaria lives. This burrowingmode implies the existence of a tiny foot, which leaves roomfor other organs within the mantle cavity. This, together withanterior divarication of the valves permits a large volume ofwater to be ejected from the mantle cavity, but, in the caseof M. arenaria, also the existence of an enormous stomach, possiblyas an adaptation for food processing. (Received 12 April 1996; accepted 2 October 1996)     

Appearance of morphological novelty in a hybrid zone between two species of land snail

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.     

Early evolutionary trends in ammonoid embryonic development

During the Devonian Nekton Revolution, ammonoids show a progressive coiling of their shell just like many other pelagic mollusk groups. These now extinct, externally shelled cephalopods derived from bactritoid cephalopods with a straight shell in the Early Devonian. During the Devonian, evolutionary trends toward tighter coiling and a size reduction occurred in ammonoid embryonic shells. In at least three lineages, descendants with a closed umbilicus evolved convergently from forms with an opening in the first whorl (umbilical window). Other lineages having representatives with open umbilici became extinct around important Devonian events whereas only those with more tightly coiled embryonic shells survived. This change was accompanied by an evolutionary trend in shape of the initial chamber, but no clear trend in its size. The fact that several ammonoid lineages independently reduced and closed the umbilical window more or less synchronously indicates that common driving factors were involved. A trend in size decrease of the embryos as well as the concurrent increase in adult size in some lineages likely reflects a fundamental change in reproductive strategies toward a higher fecundity early in the evolutionary history of ammonoids. This might have played an important role in their subsequent success as well as in their demise.     

Finite element analysis of a double membrane tube (DMS-tube) and its implication for gastropod shell morphology

Molluscan shells, including those of Gastropoda, are formed by accretionary growth at the mantle edge. The mantle is a thin membrane of skirt-like shape, which extends minutely beyond the aperture, and its edge adds a shell increment to the aperture margin so that each increment copies a configuration of the mantle edge at that time. Thus, regulation of shell morphogeny is almost equivalent to the factors which control the mantle form at the moment of shell growth. Form of the mantle skirt is considered to be kept in a state of balance between the force of its internal stress and forces acting on it such as fluid pressure or muscle contraction. The expansion behavior of the mantle skirt has been numerically analyzed by using an elastic model (DMS-tube), which represents the fundamental structure of the mantle tissue as a double membrane structure with internal springs (DMS). Four characteristic expansion patterns of the DMS-tube have been detected: (1) general outward expansion; (2) developing a ridge-like fold on an initial longitudinal protrusion of the tube edge; (3) drastic shift of the expanded state from a uniformly curved to an elliptical shape in outline, owing to the existence of a fixed boundary condition on the tube wall; and (4) constricted protrusion on the open region of the shell wall surrounding the DMS-tube. These results have the potential for answering the following questions relating to the morphogenesis of gastropod shells. How does the mantle skirt usually make contact with the inner surface of the shell wall so as to ensure continuous accretion of shell materials to the aperture margin? What is the cause of spiral ridges? Why do open coiling or minimally overlapping shells have generally circular apertures, while shells with apertures overlapped by whorls have non-uniformly curved apertural lips? What is the cause of long closed spines and why do they always appear on spiral ridges?