Paleontology Zorritos Formation 145 



instead of transversely. The outline and general shape 

 appear to be close. T. hcrvidcrana differs from such forms 

 as T. caroll'icnsis Dall ^'^^ from the Miocene of Xorth 

 Carolina, chiefly in being larger and heavier, with the 

 characteristic anterior lateral tooth. It is not very close. 



Toula ^®2 lists a form from the Gatun as Callista ( ?) 

 (Dionc) sp. which may be a Transemiella somewhat similar 

 to T. hcn'ideraua, though even larger. It is described from 

 a cast with fragments of the shell remaining, and since the 

 hinge is not preserv^ed a generic determination is not possi- 

 ble, but the contour of the shell is evidently similar. 



Lower Zorritos. Hervideras. 



Genus Macrocallista Meek 



MacrocaUista helcnae n. sp. 



Plate IX — Figs. 3, 4. 



Shell medium-sized, thin, somewhat delicate, produced 

 posteriorly. Anterior margin rounded, very little produced; 

 ventral margin broadly elliptical ; posterior dorsal margin 

 almost straight, the posterior marginal angle acute. Surface 

 uniformly convex, excepting the impressed areas, with a 

 long keel extending from the beak to the posterior marginal 

 angle. Umbones strongly prosog\rate. situated very near 

 the anterior end, proximate. Sculpture of concentric lamel- 

 lae, the sharp edges directed upward, about 4 in the space 

 of 5 mm. on the posterior slope near the margin. Areas 

 rather deeply impressed ; the escutcheon is lanceolate, 

 bounded by a distinct ridge, and the lunule is small, of 

 elongated heart-shape. Hinge with a prominent bifid right 

 anterior cardinal tooth. Inner shell not known. Length, 

 32: height, 26.5: diameter, 15.5 mm. 



This species is very close to Macrocallista conradiana 



Trans. Wag. Inst., vol. 3, pt. 6, p. 1242. pi. 55. fig. 4, 1903. 

 'Jahrb. k.k. geol. Reichs. Bd. 58, p. 726, pi. 28. fig. 9, 1908. 



