The Genus Stigmatocythere from the Quilon Beds (Lower Miocene) of Kerala, India

Abstract

Nine species of the genus Stigmatocythere Siddiqui, 1971, including six new, are described and illustrated from the Quilon beds (Lower Miocene) of Kerala, India. Of these, four species have normal overlap and hinge structure as given by Siddiqui, while the other five species show reversal of overlap and hinge structure. In the latter species the right valve is larger than the left valve, and anterior and posterior teeth, anteromedian socket and posteromedian groove are present in the left valve, and complementary hinge structure in the right valve. These species constitute a distinct group and deserve a separate designation in odrer to differentiate them from the species with normal overlap and hinge structure. Accordingly a new subgenus Bhatiacythere is erected to accommodate the former group of species and the latter species are assigned to the subgenus Stigmatocythere s. s. The checklist of the species recorded in this paper is as follows: Stigmatocythere (Stigmatocythere) chaasraensis (Guha, 1961), S. (S.) latebrosa (Lyubimova and Guha, 1960), S. (S.) multicostata n. sp., S. (S.) quilonensis n. sp., Stigmatocythere (Bhatiacythere) arcuata n, sp., S. (B.) interrupta n. sp., S. (B.) rete n. sp., S. (B.) reversa Khosla, 1976, and S. (B.) spinosa n. sp. Biostratigraphical and geographical distributions and evolutionary relationships of the above species are also discussed in the paper.