PALEONTOLOGY ZORRITOS FORMATION 151 



impressed, and longitudinally striate; it is heart-shaped, 

 and fairly broad, with the median line (juncture of the 

 valves) raised. In young specimens the elevation of the 

 margins in the lunule is more prominent. The escutcheon 

 is narrow, lanceolate, and V-shaped in cross-section; it is 

 deep and finely striate longitudinally. Hinge not known. 

 Length, 31; height, 28.5; diameter, 14 mm. (young speci- 

 men) ; estimated dimensions of full-grown specimen, length, 

 65 ; height, 56 ; diameter, 28 mm. 



All of Nelson's specimens of the species are considerably 

 worn, most of them amounting to little more than casts 

 with the inner layer of shell remaining, and those of the 

 Hopkins collection, which are better preserved, are identi- 

 fied as the same species after comparison of the sculpture 

 with fragments of that remaining on some of Nelson's 

 specimens. Chione variabilis is not far removed from 

 C. angelana, of the Upper Zorritos, having similar sculpture 

 and agreeing in other general features except the shape and 

 the area. C. angelana is more elongate both anteriorly and 

 posteriorly, and its lunule is much more deeply impressed, 

 more perfectly heart-shaped, and not raised at the juncture 

 of the valves. The anterior dorsal margin ascends more 

 abruptly in angelana. 



C. walli Guppy, 170 from Manzanilla, Trinidad, has similar 

 concentric sculpture, and the presence of the paired radial 

 ribs on the disc emphasized by Guppy in his description of 

 walli serves further to mark the similarity. Guppy's figure 

 shows walli to be more produced posteriorly and apparently 

 not as highly inflated as variabilis; this difference, together 

 with the fact that variabilis is much larger, and has slightly 

 more closely set concentric lamellae, separates the two 

 species. 



Upper Zorritos. Quebradas del Grillo and del Toro. 



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



