On a New Exogyra from Del Rio Clay 11 



ones ; the ribs mostly are destroyed by erosion and the shell 

 becomes entirely smooth, the beak detached from the body. 

 It brings to mind at once the elongate varieties of Ex. pon- 

 derosa but notwithstanding the great similarity between the 

 two species I have become convinced that they should be 

 separated specifically on account of the somewhat different 

 form of the younger specimens, the generally broader shape 

 of the mature Ex. ponderosa, and the extremely different 

 age of the beds in which both species occur. 



Those small individuals from Cerro de Muleros which I 

 have taken to be juvenile forms of Ex. Whitney i show an 

 astounding similarity to a small Exogyra, extremely fre- 

 quent in the Mainstreet limestone of Denison. This form 

 has been frequently cited under the name of Exogyra arie- 

 tina, but is easily distinguished from it by its rather strong 

 ribs and the form of the beak pressed against the body of 

 the shell. The real Ex. arietina occurs in a somewhat higher 

 horizon of the same locality ; that is to say, in the Grayson 

 Marls. The similarity between the small Exogyra of the 

 Mainstreet limestone and the small individuals from the red 

 sandstone of Cerro Muleros is so great that I have become 

 somewhat doubtful if these latter forms are really the ju- 

 venile forms of Ex. Whitneyi and not perhaps a different 

 species identical with or very similar to the small Exogyra 

 in the Mainstreet limestone, although they certainly occur 

 together with Ex. Whitneyi in the red sandstone of Cerro 

 Muleros. It is of course possible that the small Exogyra 

 of the Mainstreet limestone is at the same time a prede- 

 cessor of Ex. Whitneyi and of Ex. arietina. 



A species very nearly related to Ex. Whitneyi is Ex. 

 ferox, if it is not only a variety. Ex. ferox was described 

 by Cragin 1 ; unfortunately his figures are so deficient that 

 one cannot get an adequate idea of the shape, and the upper 

 valve of the original has been lost. The lower valve is very 

 similar to that of the mature Ex. Whitneyi, but is still more 

 elongate; no closer comparison can be made before the ju- 



iCragin, loc. cit., p. 185, pi. 32, fig. 1; pi. 33, fig. 5; pi. 34, fig. 1; 

 pi. 36, fig. 6. 



