REMOTE BIOMINERALIZATION IN DIVARICATE RIBS OF STRIGILLA AND SOLECURTUS (TELLINOIDEA: BIVALVIA)

Deposits composed of aragonite prisms, which were formed afterthe outer shell layer, have been found at the posterior steepslopes of divaricate ribs in two species of Strigilla and anothertwo of Solecurtus. These prisms have their axes oriented perpendicularto the outer shell surface and differ in morphology from fibresof the surface-parallel composite prisms forming the outer shell.They display crystalline features indicating that, unlike crystalsforming the outer shell surface, their growth front was free,unconstrained by the mantle or periostracum. These particulardeposits are called free-growing prisms (FGPs). In these generathe periostracum is clearly not the substrate for biomineralizationand, upon formation, does not adhere to the steep slope of ribs,but detaches at the rib peak and reattaches towards the posterior,just beyond the foot of the posterior scarps of ribs. In thisway, a sinus or open space developed between the internal surfaceof the periostracum and the outer shell surface along each steeprib slope. These spaces could remain filled with extrapallialfluid after the mantle advances beyond that point during shellsecretion. FGPs grow within this microenvironment, out of contactwith the mantle. Other species with divaricate ribs do not developFGPs simply because the periostracum adheres tightly to both ribslopes (which are never so steep as in Solecurtus and Strigilla).FGPs constitute one of the rare cases of remote biomineralizationin which aragonite is produced and direct contact with the mantlenever takes place. (Received 22 November 1999; accepted 20 February 2000)