10 University of Texas Bulletin 



the ammonites, represents the very uppermost part of the 

 Cenomanian, several Exogyras occur which are larger than 

 most of those in beds below, with only the possible excep- 

 tion of Ex. americana. 



The species found in the Buda limestone is Ex. Clarki 

 Shatt., 1 but the specimens figured by Shattuck represent 

 only medium-sized individuals and the species should be re- 

 described and figured. It is a very broad and not very 

 thick species very similar to Ex. americana; the medium- 

 sized specimens show rather strong radial ribs, while the 

 mature specimens are almost smooth. Very juvenile speci- 

 mens do not seem to have been found. The species evi- 

 dently is a direct descendent of the older Ex. americana. 



In the corresponding strata of the Cerro de Muleros an 

 Exogyra is found in great numbers, which I took formerly 

 to be identical with Ex. Clarki and which on account of its 

 great similarity with Ex. ponderosa I described as Ex. pon- 

 derosa var. Clarki. 2 The medium-sized specimens of this 

 species really are extremely similar to Ex. Clarki while the 

 full grown individuals are entirely different. I therefore 

 rename the species from the Cerro de Muleros and call it 

 Exogyra Whitneyi in honor of Prof. F. L. Whitney, whose 

 beautiful studies about the echinoid and mollusk fauna of 

 the Buda limestone have given us the first idea of its rich 

 fauna and its real age. 



I have figured especially the juvenile and medium-sized 

 specimens of Ex. Whitneyi in the work cited above. If we 

 compare figs. 5, 6, 7, 11 on plate 26 with specimens of 

 Ex. plexa, we can find nearly no difference. Comparing 

 figs. 4, 8, 10, 12 on the same plate with Ex. Cartledgei and 

 Ex. Drakei we see at once that the similarity is extremely 

 great, although the beak is not quite as much coiled spirally 

 as in Ex. Cartledgei. The upper valve figured in fig. 9 of 

 the same plate is very similar to that of Ex. Cartledgei. 

 The mature forms are entirely different from the younger 



iShattuck, Buda Limestone, Bull. U. S. Geol. Surv. no. 205. 1903. 

 p. 22, pi. 10, 11. 



2 E. B6 se , Cerro de Muleros, p. 115, pi. 23, fig. 17; pi. 24, fig. 6; 

 pi. 25, fig. 8; pi. 26, fig. 4-11. 



