150 . Studies in Geology, No. 3 



Dominican Republic is a similar, thoug-h smaller, form. In 

 sculpture and general outline it shows little difference, but 

 it appears to be more rounded posteriorly than aequicincta, 

 and the lunule is certainly more clearly defined. P. labremia 

 Maury,^**^ from the upper Oligocene ( ?) of Trinidad, ap- 

 pears similar according- to the figure, but differs in size, 

 being smaller, and in sculpture, the ribs being more widely 

 separated than in aeqiiicincta. 



Upper Zorritos. Quebrada de las Alturas. 



Subfamily VenERINAE 



Genus Chione Megerle von Miihlfeld 



Section Chione s.s. 



Chione (Chione) zwiabilis Nelson 



Plate IX— Figs. 8, 9. 



Chione variabilis Nelson, Trans. Conn. Acad., vol. 2, p. 202, 1870. 



Shell large, fairly thick, heavy, slightly oblique. Anterior 

 dorsal margin ascending somewhat from end of lunule; 

 anterior margin broadly rounded, ventral margin similarly 

 curved. Posterior margin slightly angulate. Umbones 

 inflated, prosogyrate, proximate. Surface excavated before 

 umbones, elsewhere smoothly convex, and in the region of 

 the beaks almost globose in some specimens. Concentric 

 sculpture of closely spaced ridges, the under surface of 

 which is noded, each node being sulcate in the center of 

 the disc. Radial sculpture of closely spaced ribs, somewhat 

 obscure in all specimens available, but some worn specimens 

 show a pairing of the ribs on the disc, with one less prominent 

 rib between the pairs. In much worn specimens all traces of 

 most of the concentric lamellae are gone, but some, more 

 distantly separated, remain, giving the appearance of a 

 different sculpture. There is a distinct lunule, somewhat 



'Jour. Ac. Nat. Sc. Phila., vol. 15, p. 57, pi. 9, figs. 14, 15, 1912. 



