On a New Exogyra from Del Rio Clay 5 



line to immature specimens of Ex. texana, with 5 to 8 radial 

 folds on the anterior slope, a distinct umbonal ridge, and a 

 steep anterior slope. The upper valve is practically smooth. 

 This form seems to be intimately related to Exogyra Bous- 

 singaulti d' Orb. 1 which occurs approximately in the cor- 

 responding horizon of Europe. A similar form has been 

 found in the Aptian of San Juan Raya, Puebla, Mexico. 



This species seems to have branched off into several forms 

 in the next higher division of the Trinity formation, some 

 of which have been described and very imperfectly figured 

 by Cragin as Ex. paupercula 2 and Ex. weather for densis.* 



These species are quite frequent in the Alternating beds 

 (Glenrose), the middle and upper part of the Trinity form- 

 ation. Both seem to be related to Ex. Hilli and Ex. Bous- 

 singaulti, although the distinguishing feature in Ex. pau- 

 percula is that the beak becomes free from the rest of the 

 shell as in Ex. arietina. Ex. paupercula has only 2 to 4 

 irregularly-topped, angular, radial folds on the lower valve, 

 while the upper valve seems to be smooth. I have not been 

 able to see the original specimens. 



Much more similar to the Ex. Boussingaulti group is the 

 Ex. weather f or densis (pi. 1, fig. 1, 2) with a strong um- 

 bonal ridge, 6 to 12 oblique plications on the anterior slope, 

 and a greater number of narrow, subradiate, or curved pli- 

 cations on the posterior slope. The upper valve is smooth, 

 but the margin is often scalloped corresponding to the end- 

 ing of the ribs on the lower valve. 



This is, at least, what I take to be Ex. weather for densis, 

 a species which has not been very well figured. In the 

 collection of the University of Texas exists a number of 

 specimens collected by Taff at one quarter of a mile west of 

 Weatherford, labeled E. Boussingaulti d'Orb. This species 

 certainly is very similar to the French form and a near rela- 

 tive of it, and at the same time also of Ex. Hilli. It does 



1 A.d' Orbigny, Voy. dans 1'Amerique merid. t. Ill, 4me part, Pal., 

 p. 91, pi. 18, fig. 20; pi. 20, fig. 8, 9. 



Idem, Paleont. frang., Terr. Cret. t. Ill, p. 702, pi. 468. 

 2 Cragin, loc. cit., p. 186, pi. 30, fig. 7, 8. 

 3 Idem, ibidem, p. 188, pi. 45, fig. 7-10. 



