16 The Fauna of the Keyser Member of the 



Superfamily PENTAMERACEA 

 Family PENTAMERIDAE 



Genus GYPIDULA Hall 



Gypidula (Sieberella) coeymanensis var. prognostica n. var. 



Plate LXII, Figs. 9-11 



Description. — Shell subovoid to longitudinally subglobose, length 

 greater than the breadth. Ventral valve convex, quite gibbous to sub- 

 ventricose; beak pointed, arched and strongly incurved, but not closely 

 incurved over that of the opposite valve. Dorsal valve convex, subcircular 

 to transversely elliptical, slightly gibbous in the umbonal region; beak 

 incurved, extending to that of the opposite valve and beneath it. Surface 

 marked by rather indistinct plications which are more conspicuous from 

 the center towards the anterior. From the center to the beak the shell is 

 smooth except in the very large individuals where plications extend beyond 

 the center becoming less conspicuous posteriorly. Very fainr concentric 

 lines of growth mark both valves. There is no distinct fold or sinus on 

 either valve. Internally the dental lamellas converge to form a triangular- 

 shaped cavity which forms a continuation from the triangular delthyrium 

 beneath the beak of the ventral valve. A central septum extends from the 

 beak of the ventral valve more than two-thirds the length of the shell 

 almost to the front. In its posterior portion the central septum is united 

 to the converging dental lamellae beneath the triangular aperture, formed 

 by the delthyrium and the dental lamellae. The dorsal interior not seen. 

 Dimensions vary from small individuals 11 mm. long and 10 mm. wide to 

 individuals 26 mm. long and 25 mm. wide. 



Mature individuals of this variety are distinguished from those of 

 Gypidula coeymanensis by the truncated anterior margin, less inflated beak 

 of the ventral valve which is only slightly incurved over the beak of the 

 brachial valve, by the feebly developed sinus and fold, rather indistinct 

 plications and small size. The adult forms of this variety resemble more 

 or less closely some of the immature shells of G. coeymanensis. This 

 variety abounds in a zone a few feet thick near the middle of the Keyser 

 member at most localities at which its horizon is exposed. It is hence 



