On a New Exogyra from Del Rio Clay 7 



moves nearer to the anterior margin, which becomes ex- 

 ceedingly steep, while the posterior slope becomes flattened 

 or even concave. The upper valve develops a great number 

 of somewhat irregular radial ribs, the umbonal ridge be- 

 comes very pronounced and the anterior slope quite steep. 

 A conscientious study of the forms from different horizons 

 may show that even within the species a certain develop- 

 ment takes place, in so far as the umbonal ridge is nearer 

 to the anterior margin in the older specimens than in the 

 younger ones, and that the ribs grow coarser in the indi- 

 viduals found in the higher horizons. 



Ex. texana nearly always occurs in great numbers and 

 with it begins the rich development and subdivision of the 

 principal tribe. 



Before we go any farther we shall have to mention an- 

 other form developed during the; Albian or the Vraconnian 

 and apparently the ancestor of several younger species. 

 This is the form described by Cragin 1 as Ex. plexa. Under 

 this name Cragin seems to have united at least two differ- 

 ent species; I refer his name to one of these which looks 

 entirely like an enlarged juvenile specimen of Ex. arietina 

 (compare the description of this species given below) . The 

 exact types could not be found, but there is sufficient ma- 

 terial from the original localities. (Compare pi. 1, figs. 3, 

 4.) I regard as the type the specimen figured by Cragin 

 in his figures 3, 5, 6 (not 4). It is entirely covered with 

 fine radial ribs which cross the prominent umbonal ridge 

 obliquely as in the juvenile specimens of Ex. arietina. The 

 specimens occur in the bed of Ex. texana below the Kiami- 

 tia clays on the Texas and Pacific railroad, three miles west 

 of Benbrook. The whole shell is spirally curved similar to 

 the young Ex. arietina although the size is much larger. 

 The beak is still pressed down against the body of the shell, 

 but a further spiral development would detach the beak 

 from the rest of the valve. 



This species has probably developed from the group of 

 Ex. Boussingaulti (Ex. weatherfordensis) losing the thick 



iCragin, loc. cit., p. 187, pi. 30, fig. 3-3. 



