Paleontology Zorritos Formation 123 



outline. The ribs of A. imporcata are wider than the inter- 

 spaces : the reverse is true of A. torocnsis. There is consid- 

 erable variation in outline in this species, and it was con- 

 sidered advisable at first to divide the group more finely, 

 giving such differences specific value, but the intergrada- 

 tion of the characters and their irregular overlapping in 

 various specimens led to the "lumping" of all forms which 

 show constancy in the major characters. Two mutants, the 

 varieties crassa and prolata, show sufficient variance from 

 the form considered as the type to require at least varietal 

 distinction. 



Area chiriquiensis Gabb,^-'^ from the Miocene of Santo 

 Domingo and the Oligocene (?) of Chiriqui. is very closely 

 related. The ribs of toroensis are not as thick as those of 

 chiriquiensis. and are less coarsely noded : the hinge-line is 

 normally longer than that of eliiriquicnsis, and the vertical 

 distance between the hinge and the ventral margin is smaller. 

 Otherwise the two tally closely. A. anguillana ^-^ Cooke, 

 from the Oligocene of Antigua, is exactly similar in shape, 

 and differs only in size and in the area, which in toroensis is 

 not bounded by a marginal elevation, has a larger number 

 of more irregular grooves, and is longer. 



There is striking resemblance between the more rounded 

 specimens of A. toroensis and A. idonea Conrad ^-' from the 

 Miocene of Maryland. \'irginia. and Florida. The only 

 differences between the two lie in the ribs, which are some- 

 what tuberculated and more rounded in A. toroensis, and the 

 area, which is uniformly higher, with a narrower limiting 

 angle. 



Upper Zorritos. Ouebradas del Toro, Tijeritas and del 

 Grillo. 



^Manry. Bull. Amer. Pal., vol. 5. Xo. 29. p. 174. pi. 28. fig. 2, 

 1917. 

 ""Carnegie Inst. Wash., publ. 291. p. 127, pi. 5. figs. loa, b, 1919 

 '^Md. Geol. Surv.. Miocene, p. 389. pi. 106. figs, i, 2. 1904. 



