836 



HISTORICAL GEOLOGY. 



1367 



1368 



Vertebrates. — Some scales of Ctenoid fishes have been found in the 

 Potomac beds. But the Vertebrates of special interest are the large Keptiles : 



a species related to the 

 1367-1368. Morosaurus, the Astrodon 



,7b7m.sio?iu' of Leidy (1865); 

 and the other Dino- 

 saurs Pleurocoelus nanus, 

 P. altus, Priconodon cras- 

 sus, AUosaurus (?) medius, 

 and Coeluras gracilis, de- 

 scribed by Marsh (1888). 

 Fig. 1367 represents a 

 side view of one of the 

 dorsal vertebrae of Pleu- 

 rocoelus nanus, and 1368, 

 an inside view of a tooth 

 of Pricoyiodon crassus. On account of the Jurassic features of the Reptiles, 

 the Potomac group has been referred by Marsh to the Upper Jurassic. 



From the Lower Cretaceous of Texas and its continuation into Oklahoma 

 (formerly Indian Territory) five species of Pycnodont Fishes have been 

 described by Cope : Mesodon diastematicus, M. Dumhlei, and two species of 

 Uranoplosus and one of Coelodus. 



Dinosaurs. - 



• Fig. 1367, Vertebra of Pleurocoelus nanus ; 136S, tooth of 

 Priconodon crassus. From Marsh. 



Characteristic Species. 



The fauna of Texas (and the country beyond to Mexico) has special interest, because 

 the region is the only one of the Lower Cretaceous in North America abounding in marine 

 fossils. The characteristic sjpecies are as follows, according to Hill : 



1. Trinity group. — The Glen Hose beds have afforded: Ostrea Franklini Coqaand, 

 Modiola Branneri Hill, Pecten Stantoni Hill, Requienia Texana, Barhatia parva Missouri- 

 ensis, Isocardia medlalis Conrad, Natica pedernalis Roemer, Nerinea Austinensis 

 Roemer; also, Crocodiles, Dinosaurs, Chelonians, and Fishes not yet studied. A bed of 

 chalk is composed of the Rhizopod Patellina (Orbitulites) Texana R. (Fig. 1357). 



2. Fredericksburg group. — The prominent fossils of its several subdivisions are the 

 following: (1) The Gryphoea rock and Wcdnut sands: Exogyra Texana R. { — E. 

 flabellata Goldfuss) ; and, higher up, a bed made up of Gryphcea Pitcheri (the small form 

 figured by Conrad). (2) The Comanche Peak chalk: Pseudodiadema Texanum R:, 

 Enallaster Texanus R., Exogyra Texana, Gryphcea Pitcheri Conrad (not Marcou), Janira 

 occidentalis Con., Protocardium Hillanum Sowerby, Nerinea acus R., Ammonites 

 {Buchiceras) pedernalis R. (3) The Caprina limestone, also called the " Hippurite " 

 limestone: Nerinea Austinensis R., N. cidtrispira R., iV. subida R., Cerithium 

 Austinense R., Trochus Texanus R., Solarfum planorbis R., 3Ionop>leura marcida White, 

 31. pinguiscida White, Bequienia patagiata White, Ichthyosarcolithes (Caprina) anguis 

 R., I. (?) crassifibra R., I. (?) planatus Con., Badiolites (Sphcerulites) Texanus R. 



3. Washita group. — (1) The Preston beds, Schloenbachia clays, including lime- 

 stone flags, Gryphcea fornicidata White ( — G. Pitcheri Marcou), and the Ammonite 

 Schloenbachia Peruviana v. Buch. ; the limestone is the building material of old Fort 

 Washita. (2) The Duck Creek chalk, many Ammonoids, among them Pachydiscus 

 Brazoensis Shum., Schloenbachia Belknapi Marcou, and Hamites Fremonti Marcou ; with 



