ZORRITOS FORMATION 153 



forms a distinct angle with the ventral margin, and is almost 

 straight. Umbones prosogyrate, anterior in position. Gen- 

 eral surface markedly convex ; a pronounced posterior 

 carina separates the narrow posterior surface of the shell 

 from the rest. Concentric sculpture of the prominent lamel- 

 lae typical of the group, distinctly spaced, crenulate on the 

 under surface. Radial sculpture of ribs which show a some- 

 what indistinct yet recognizable pairing on the disc; there 

 are less prominent ribs between the pairs. Lunule not clear- 

 ly visible on specimens studied, but it appears to be im- 

 pressed and configured much as in C. walli Guppy, C. wood- 

 wardi Guppy, and related species. The escutcheon is long, 

 somewhat broad. Inner margin finely crenulate. Length, 

 24; height, 21.5; (semi) diameter, 8.5 mm. 



The difference between this species and C. walli Guppy, 171 

 from the Oligocene ( ?) of Trinidad, is small. The Trini- 

 dad form has a greater number of more closely spaced 

 concentric lamellae, and the pairing of the radial costae is 

 apparently more prominent. Otherwise differences are small 

 and not constant. C. woodwardi Guppy 172 is also similar, 

 but lacks the pairing of the ribs, which are obviously stouter. 

 In shape the forms are similar. Maury has described 173 

 as Chione cf. walli a form which she states to show the 

 pairing of the ribs characteristic of walli, but her figure 

 indicates rather the absence of the feature, and there is 

 small doubt that the specimen figured is C. woodwardi. 



C. spenceri Cooke, 174 from the Oligocene of Antigua, is 

 of the same general type, but differs in having weaker 

 radial ribs, particularly distally, and, apparently, in lacking 

 the paired ribs. 



171 Quart. Jour. Geol. Soc. Lond., vol. 22, p. 581, pi. 26, fig. 16, 1866. 



172 Idem., p. 292, pi. 18, fig. i; Maury, Bull. Amer. Pal., vol. 5, 

 No. 29, p. 218, pi. 37, fig. 6, 1917. 



173 Bull. Amer. Pal., vol. 5, No. 29, p. 218, pi. 37, fig. 7, 1917. 



174 Carnegie Inst. Wash., Publ. 291, p. 150, pi. 15, figs. la, b, 1919. 



