42 COLORADO COLLEGE STUDIES. 



Terehratella toacoensis, Roem. Roudairia denisonensis, Crag. 



Ostrea franklini, Coquand. Pholadumya ragsdalei. Crag. 



Ostrea suhovata, Shuni. Pholadomya sancti-sabai, Roem. 



Ostrea quadrupUcatiu Shum. Homomya loashita, Crag. 



Exogyra arietina, Roem. Pavhymya austinensis, Shum.f 



Lima ivacoensis, Roem. Tuvritella denisonensis, Crag. 



Vola texana, Roem. Turritella seriatim-granulata, R. 

 Trigonia clavigera, Crag. (?)* 



All of the above-listed fossils were obtained by the writer 

 in this limestone at Denison, excepting the Holectyjms, 

 which was recorded from the same limestone at Cedar Mills 

 (Taff) in the writer's "Contribution to the Invertebrate 

 Paleontology of the Texas Cretaceous." 



Perhaps the most marked paleontological characteristic of 

 the Choctaw limestone is the plentiful occurrence of Tere- 

 hratella wacoensis in association with Exogyra arietina. In 

 Grayson and Cooke counties, a good guide-fossil is Phola- 

 domya ragsdalei, whose peculiar ornamentation usually suf- 

 fices, even in fragments of casts or moulds, for its recogni- 

 tion. This fossil has hitherto been found in the Choctaw 

 limestone only, and is by no means rare. 



In the Arietina marl of more southerly counties, as Wil- 

 liamson, Travis, and Hays, the Choctaw terrane is repre- 

 sented by the calcareous transition from the marl of the me- 

 dial, culminating horizon of Exogyra arietina to the Wash- 

 ita limestone below, and its fauna is poorer and less peculiar. 

 There, as in northern Texas, Terehratella wacoensis and 

 Exogyra arietina are the leading members of its fauna, the 

 others being such as are common in the upper part (Denton 

 beds]:) of the Washita limestone, such as Vola texana, Tiir- 

 rilites brazosensis, etc. 



* The only specimen of Trigonia that I liave soon in the Choctaw limestone is a 

 cast of a small one, that does not show the cliaiactor of the ornamentation, but 

 agrees in size and form apparently with T. clavif/cra, and is probably that species. 

 T. clavigera is the common Trigonia of tlie Denton marls, and resembles T. emoryi, 

 with which Hill has apparently confused it, only in badly worn specimens. Com- 

 paring, in numbers, finely preserved specimens of T. clavigera from the Denton 

 marls of Cooke county, Texas, and the Chickasaw Nation, with such of T. emoryi 

 from the Walnut clays, confusion of the two species is impossible. T. crenulata, 

 cited by Taff from the Denton marls, does not occur in rocks of the ("omanche series. 



t Not ""Pachyma" as it is often spelled and pronounced. 



1 The Denton marl was clearly defined as to position, paleontology, and typical 

 locality by Taff, by means of a detailed section, in the Fourth Annual Report of the 

 Geological Survey of Texas, Part I, p. 272, and, though he failed to apply it elsewhere 

 in the same report, this name has priority over tiie later name, "Marietta" proposed 

 by Hill for the same terraue. 



