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 ectepicondylar 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 labyrinthine teeth and the absence of a pubo-ischiatic opening. The arrangement of the bones of the temporal region is doubtful. The legs are essentially terrestrial in structure, with but slight aquatic adaptations, but the heavy flattened ribs and the elongate flattened 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. Simœdosaurus 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, without foramina.
These small reptiles of but three or four feet in length are still imperfectly 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 mosasaurs; the dentition intermediate between that of the Mosasauridae and that of the Globidentidae.
