On a New Exogyra from Del Rio Clay 9 



americana Marcou. Unfortunately, only very large speci- 

 mens are described and figured, but the individual illus- 

 trated by Marcou shows that the younger specimens must 

 have strong, fold-like, radial ribs in the umbonal region. 

 Therefore I presume that this broad and flat species with 

 its obtuse ridge near the anterior margin developed from a 

 broad, coarsely-ribbed group, probably one derived from 

 Ex. texana. 



Exogyra becomes exceedingly frequent on top of the 

 Georgetown formation in the Del Rio clay which represents 

 part of the upper Cenomanian. 



The principal species is Ex. arietina Roem. which we have 

 redescribed farther on. It is evidently a descendent of Ex. 

 plexa; it retains the ornamentation of this species and its 

 form on the umbonal region and in the very young indi- 

 viduals, but later on becomes entirely smooth and its beak 

 becomes detached because the whole valve coils spirally. 



A second species is Ex. Cartledgei nov. sp., described be- 

 low. It certainly was derived from Ex. nov. sp. aff. plexa, 

 developing the strong radial ribs still more, while the beak 

 became somewhat detached from the rest of the shell by the 

 coiling of the valve. The strong umbonal ridge is still in 

 existence, but the fine ribs on the beak were lost ; or at least 

 are not preserved on the specimens I have in hand. This 

 species grows much larger than either Ex. nov. sp. aff. 

 plexa or Ex. arietina. 



Cragin 1 described another species from the Del Rio clay 

 which he called Ex. Drakei. Unfortunately his figures are 

 very deficient, but his originals show that the species is 

 entirely different from Ex. Cartledgei. It is covered with 

 irregular but strong ribs on the lower valve, not really 

 coiled, but rather bent over to one side, as shown in Cragin's 

 figure 11. The upper valve is very similar to that of Ex. 

 arietina and Ex. Cartledgei. 



In the next higher horizon, the Buda limestone and the 

 corresponding strata of the Cerro de Muleros, which, ac- 

 cording to the echinoid fauna described by Whitney, and 



in, loc. cit., p. 184, pi. 29, fig. 8-11. 



