On a New Exogyra from Del Rio Clay 13 



possible that Ex. laeviuscula Roemer or a related form oc- 

 curs there, as it certainly does in the middle portion. This 

 very characteristic form is entirely smooth, but if the 

 younger specimen figured by Roemer in PL 9, fig. 3c, really 

 belongs to this species, we might suppose that it is derived 

 from Ex. nov. sp. aff. plexa through Ex. Cartledgei and 

 Ex. cohimbella Cragin (not Meek) having lost entirely the 

 finer ribs. 



An undescribed and unstudied small Exogyra is very fre- 

 quent in the middle part of the Austin Chalk (zone of 

 Mortoniceras texanum) on the Rio Grande. 



In the upper part of the Austin Chalk near Austin we 

 find a great number of a large Exogyra which commonly 

 has been called Ex. ponderosa, although it is specifically 

 quite different. It has a broad and not very convex lower 

 valve, with a rounded ridge and an extremely steep slope 

 on the anterior margin, and is very similar to the large 

 specimens of Ex. Clarki and to a certain degree to Ex. amer- 

 icana. These species with very low right valves probably 

 form an independent branch and are derived from Ex. tex- 

 ana. I have not seen any of this tribe in higher strata of 

 Texas, but a near relative, Ex. tamulica Stol., appears in 

 the higher Senonian of India. 



The Exogyras of the higher strata of Texas, the Taylor 

 marls (upper Santonian), Navarro beds (Campanian) and 

 the higher Escondido beds (Maestrichtian) are very little 

 known, practically only two species having been described : 

 Ex. ponderosa Roem. from the Taylor marls and Ex. costata 

 Say from the Navarro beds, which seems to continue into 

 the base of the higher Escondido beds. 



Exogyra ponderosa has been first and very well described 

 and figured by Roemer. 1 In recent years it has been re- 

 described and figured by Stephenson. 2 His figures are 

 very characteristic but unfortunately he has not tried to 

 describe and figure the development of the species from the 



Roemer. Kreidebild. v. Texas, p. 71, pi. 9, fig. 2. 

 2 L. W. Stephenson, Cret. dep. on the Eastern Gulf region. U. S. 

 Geol. Surv. Prof. Paper 81, 1914, p. 46, pi. 13, fig. 5-7; pi. 14; pi. 15, 



rig. 1-3. 



