On a New Exogyra from Del Rio Clay 19 



This seemingly depends on the thickness of the valve. To- 

 ward the interior and posterior side of the groove, a striated 

 protuberance elevates itself more or less highly ; it is oppo- 

 site the depression formed by the striated shelf and cren- 

 ulated margin of the lower valve, and probably 'fits into it. 

 The muscular impression is deep and near the posterior 

 side, far below the region of the beak. 



Relation to other species: 



Ex. Cartledgei has a certain similarity to Ex. arietina, 

 but its strong ribs and large size make it easy to distinguish 

 the two. It probably has been derived from the same tribe, 

 that of Ex. plexa, or a similar form, as has been explained 

 in the first chapter of this paper. 

 Occurrence: 



The species has been found very frequently in the Del 

 Rio clay in a horizon about 10 to 30 feet below the Buda 

 limestone. The Del Rio clay has at this place a thickness 

 of 120 feet or more. It is associated with Nodosaria tex- 

 ana, which occurs in abundance above it, Gryphaea sp., En- 

 allaster cf. bravoensis, and Hemiaster sp. According to 

 Dr. Udden, Exogyra arietina is rare at this locality but 

 was noted at a level not far from the middle height of the 

 Del Rio clay. 



Locality: 



Reed Plateau, point almost exactly one mile south from the 

 Chisos Mining Company's mine, in Brewster County, Texas. 



Exogyra arietina Roemer 

 PI. IV, fig. 1-18; PL V, fig. 1-23. 



1852. Ferd. Roemer, D'e Kreidebildungen von Texas und ihre or- 

 ganischcn Einschliisse. Bonn. P. 68, PI. 8, fig. 10. 



Roemer's description of this species is very careful and 

 his illustrations are quite good, but figures of the juvenile 

 stages of the species are lacking, as these were not given 

 overmuch importance by the paleontologists of that time. 



