On the Upper Maastrichtian Oysters of the Genus Rhynchostreon Bayle (Bivalvia,Gryphaeidae) from the Mountainous Crimea

The morphology and shell microstructure of smooth exogyrid oysters from the Upper Maastrichtian of the Mountainous Crimea (SW Russia) are studied. Their shell microstructure consists of a simple regularly foliated, irregular crossed foliated, and transitional irregular simple prismatic/irregular complex crossed foliated structures. The latter has been only identified in oysters of the genus Rhynchostreon Bayle. Along with specific shell morphology, it allows to attribute the studied oysters to the species Rhynchostreon aralensis (Arkhangelsky, 1912). The occurrence of the genus Rhynchostreon in the Upper Maastrichtian of the Mountainous Crimea allows to clarify its stratigraphic distribution.