THE SUBCLASS DIAPSIDA 283 



C. SUBORDER CHORISTODERA 



Elongate, subaquatic reptiles, with a very slender face, terminal 

 undivided nares, with small teeth on all palatal bones. No parietal 

 foramen. Internal nares posterior. Teeth labyrinthine in structure. 

 Vertebrae shallowly amphicoelous without dorsal intercentra. 

 Twenty-six presacral, two or three sacral, and a long, flattened tail. 

 Dorsal ribs holocephalous, broad, and heavy. Parasternals stout. 

 Pelvis without pubo-ischiatic opening. Humerus with ectepicon- 

 dylar foramen. Mesopodials imperfectly known. 



This small group of water reptiles, animals reaching a length of 

 eight feet, is of interest because of the retention of several primitive 

 characters, otherwise unknown in the Diapsida, especially the laby- 

 rinthine teeth and the absence of a pubo-ischiatic opening. The ar- 

 rangement of the bones of the temporal region is doubtful. The 

 legs are essentially terrestrial in structure, with but sUght aquatic 

 adaptations, but the heavy flattened ribs and the elongate flat- 

 tened tail decisively indicate bottom-crawling aquatic habits. The 

 relationships between the known genera are very close. 



Uppermost Cretaceous and Paleocene. Champsosaurus Cope 

 {Nothosaurops Leidy), North America. Simosdosaurus Gervais^ 

 France, Belgium. 



D. ? SUBORDER THALATTOSAURIA 



Marine reptiles with elongate face, posterior[ly placed external] 

 nares, sclerotic plates, and paddle-like extremities. Premaxillary, 

 anterior, mandibular, and pterygoidal teeth conical; those of the 

 prevomers, and posterior part of maxillae and mandibles low- 

 crowned. A parietal foramen. Vertebrae rather deeply biconcave; 

 intercentra unknown. Dorsal ribs holocephalous, articulating 

 chiefly with centra. Parasternal ribs slender. Humerus short, with- 

 out foramina. 



These small reptiles of but three or four feet in length are still im- 

 perfectly known; nor is it quite certain that they have two temporal 

 openings. The upper opening occupies a peculiar position. The 

 limbs, so far as known, resemble those of the mosasaurs. The habits 

 of the thalattosaurs must have been similar to those of the mosa- 

 saurs; the dentition intermediate between that of the Mosasauridae 

 and that of the Globidentidae. 



