14 University of Texas Bulletin 



youngest to the mature individuals. Roemer mentions that 

 specimens about 2.5 cm. long show distinct, unequal, radial 

 folds which begin to grow indistinct near the margin of the 

 valve. Stephenson's PI. 13, fig. 5 shows clearly this kind 

 of ribs on the beak of a medium-sized specimen. If we 

 imagine this beak detached from the valve and in the form 

 of a young specimen, it would have exactly the shape and 

 ornamentation of a young specimen of our young Ex. Whit- 

 neyi from the Cerr.o de Muleros, and be extremely similar 

 to the young of Ex. arietina and to the mature specimens 

 of Ex. plexa. Stephenson (loc. cit. p. 49, pi. 15, fig. 4; pL 

 16, fig. 1, 2) has described a number of shells as Ex. pon- 

 derosa var. erraticostata Steph. It will be rather difficult 

 to sustain this variety because the figure of Roemer's type 

 shows very similar ribs and thus should be part of this 

 variety. 



Ex. costata has been quite frequently described and fig- 

 ured ; recently Stephenson 1 has very fully described the ma- 

 ture specimens and also has illustrated these profusely. 

 Unfortunately also in this case we lack the exact knowledge 

 of the ornamentation of very young specimens. The species 

 is possibly derived from Ex. ponderosa or at least from a 

 form of which Ex. ponderosa is a branch, but this cannot 

 be proven at the present time. Specimens should be espe- 

 cially studied in the zone where Ex. ponderosa and Ex. cos- 

 tata appear to occur together and where the real antecessor 

 of the latter species may be found. 



The preceding sketch of the evolution of Exogyra in the 

 Texas Cretaceous necessarily has to be deficient, incomplete 

 and in a great part hypothetical, but at least it shows that 

 all the species, even the largest which are practically 

 smooth, came originally from a rather small costate form; 

 and that the great diversity of forms in the upper strata is 

 simply due to the subdivision of a tribe represented at the 

 base by forms that are intimately related to each other and 

 at the same time to Ex. Boussingaulti of the European 

 Aptian. It shows also that some of the large species in the 



Stephenson, loc. cit. p. 50 pi. 16. fig. 3, 4; pi. 17, fig. 1, 2; pi. 18 j 

 pi. 19, fig. 1-4; pi. 20 :"ig. 1. 



