GEOLOGIC AND GEOGKAPHIC KANGE. 45 



WESTERN GULF REGION. 



In the western Gulf region the genus ranges from the Fredericksburg group of the Lower 

 Cretaceous to the top of the Upper Cretaceous. The species common to the eastern Gulf region, 

 however, do not occur lower than the Austin chalk. All the species and varieties occurring 

 in the Upper Cretaceous deposits of the eastern Gulf region, except Exogyra upatoiensis sp. 

 nov., are known in the Upper Cretaceous deposits of the western Gulf region. The information 

 available in the literature, that afforded by the collections in the National Museum, and that 

 obtained by the writer in his field studies, give good grounds for the belief that the range 

 restrictions in the two regions are analogous. 



In Arkansas and northeastern Texas Exogyra ponderosa Roemer and Exogyra ponderosa 

 var. erraticostata, the former hi abundance, occur in the Brownstown marl. Exogyra costata 

 Say occurs in the overlying Marlbrook marl, Nacatoch sand, and Arkadelphia clay. (See 

 PI. XII.) Exogyra costata var. cancellata var. nov. occurs abundantly in the Marlbrook marl. 



In Texas the species Exogyra ponderosa Roemer is abundant in places in the upper part 

 of the Austin chalk and in the basal beds of the overlying Taylor marl. A few specimens of 

 Exogyra ponderosa var. erraticostata have been found in the basal part of the Taylor marl. 

 Specimens hi the National Museum referable to the last-named variety were collected in 1S95 

 by Vaughan and Stanton, from beds that are regarded as synchronous with the Taylor marl, 

 near San Carlos, Presidio County, Tex. 1 These specimens (U. S. Geol. Survey coll. 1467 and 

 field No. • 297) were identified by Stanton as a variety intermediate between Exogyra costata 

 Say and Exogyra ponderosa Roemer. Exogyra costata Say is a characteristic species of the 

 Navarro and its equivalent, the " Webberville " formation, of Texas. Exogyra costata var. 

 cancellata var. nov. occurs in beds probably representing the lower part of the Navarro forma- 

 tion near Cooper, Delta County, Tex., and hi cuts of the Houston & Texas Central Railroad 

 4 and 4£ miles, respectively, east of Crandall, Kaufman County, Tex. The genus has not been 

 reported from the middle and upper parts of the underlying Taylor marl, and for this reason 

 the upper limit of the range of Exogyra ponderosa Roemer and the lower limit of the range of 



Exogyra costata Say are not known. 



MEXICO. 



One specimen of Exogyra ponderosa Roemer from Mexico is in the National Museum col- 

 lections. It was obtahied by White and Stanton from a locality across the Rio Grande from 

 Presidio, Tex. (U. S. Geol. Survey coll. 622). The beds were correlated by Stanton with the 

 Taylor marl. (See p. 48.) 



Exogyra costata Say has been described from so-called lower Senonian beds exposed in 

 the vicinity of Cardenas, State of San Luis Potosi, Mexico, by Dr. Emilio Bose, whose figured 

 specimens agree closely in the surface sculpture of the lower valve with the coarsely costate 

 variety occurring near the base of the Exogyra costata zone hi the eastern Gulf region. (See 

 PI. XVII, fig. 2.) They differ from the latter, however, hi the sculpture of their upper valves, 

 the concentric growth projections being coarser, more widely separated, and fewer hi number 

 in the Mexican specimens. Three specimens (U. S. Geol. Survey coll. 4129), which prove to 

 be the variety Exogyra costata var. cancellata var. nov., were brought from Mexico by Stanton 

 hi 1906. They were given to him at San Luis Potosi and were said to have been collected 

 near Ciudad del Maiz, State of San Luis Potosi. 



UTAH. 



In Utah, "east of Impracticable Ridge," . specimens of Exogyra have been obtained 

 from beds now known to belong to the Colorado group, and have been referred by White 2 to 

 Exogyra ponderosa (U. S. Nat. Mus. No. 13417). Although not typical, the specimens are 

 nearer to this species than to any other described form, and until better collections have been 

 made and the variations noted they may be considered correctly identified. 



i Vaughan, T. W., Reconnaissance in the Rio Grande coal fields of Texas: Bull. V. S. Geol. Survey No. 164, 1900, pp. 77-S2. 

 2 White, C A., Rept. U. S. Geog. Surveys W. 100th Mer., vol. 4, pt. 1, 1877, p. 172, PL XIV, figs. 1 a-c. 



