4 University of Texas Bulletin 



of the beak is of an importance similar to that of the suture 

 and the smaller whorls in the ammonites. 



More than twelve years ago, when I described the fauna 

 of the Cerro de Muleros near Ciudad Juarez, Chihuahua, 

 Mexico, I felt convinced that most of the Texas Exogyras 

 had developed from a common tribe, but at that time I did 

 not have enough material and there seemed to exist serious 

 gaps which in part have been filled since that date. One 

 of them especially was between the Vraconnian and the 

 highest Cenomanian; or, using local terms, between the 

 uppermost Edwards limestone and the Buda limestone. This 

 latter one has been recently filled through a discovery made 

 by Mr. W. R. Cartledge, who found a new Exogyra in the 

 upper part of the Del Rio beds in the region near the quick- 

 silver deposits of the Terlingua district; a species which 

 will be described farther on. 



I do not propose to prove that the evolution of the Texan 

 Exogyras was entirely continuous, because my material is 

 still very incomplete; but I wish to open a road for some- 

 body else who may want to demonstrate in what manner 

 Exogyra developed in Texas during the Cretaceous. It 

 will probably be found that such an evolution existed and 

 was at least partly parallel to the development of this genus 

 in southern Europe and northern Africa; possibly in the 

 later stages also in Asia. 



Thus the content of the following paper will be merely a 

 sketch, but it may point out a way for a monographical 

 description of the Cretaceous species of Exogyra in Texas.' 



The oldest species of Exogyra in the Texas Cretaceous 

 seemingly occurs in the lower part of the Trinity formation, 

 the Basement sands or Travis Peak formation of Travis 

 County. This species has been described, although net fig- 

 ured, by Cragin 1 as Exogyra Hilli. He asserts that it lias 

 also been found in Arkansas, where it was described by 

 R. T. Hill. 2 It is a small form somewhat similar in out- 



1 F. W. Cragin, A contribution to the invert. Paleontol. of the Texas 

 Cretaceous. 4th An. Rep. Geol. Surv. Texas, 1893, p. 186. 



2 R. T. Hill, The Mesozoic geology of southwestern Arkansas. An. 

 Rep. Geol. Surv. Arkansas, 1888. vol. 11, p. 131 (under the name of 

 Ostrea Franklini), pi. 5, fig. 1-10 (not pi. 5, fig. 11-18; pi. 6, fig. 

 19-27; pi. 7, fig. 28-30). 



