MERRIAM THK THALATTOSAT'TilA Zo 



spondingly greater degree a shore dweller. NectosaurnH is, however, found in the 

 same deposits with the otlier forms and appears to he as common as the others; 

 so that it is safe to eonsi<lcr it as liaviiig passed tlie greater part of its life away 

 from the sliore. 



From what we know of the vertebral eoluiiin of TlutlattDsawrm it appears 

 that the animal had a relatively short neck and a long dorsal region, the propor- 

 tions heing near tiiosc in the vertebral eolunin of some Mosasaurs. Only the 

 anterior portion of the caudal region is known. The slender, rounded neural 

 spines with well develo})e(l zygapopliyses seen hert^ are not such as commonly 

 appear in forms Avitli a highly specialized sculling tail, and it is hardly probal)le 

 that a caudal tin of hirge size was developed. 



The anterior limbs evidently formed paddles of moderate size. The pos- 

 terior pair n)ay have been larger, in compensation for lack of a strong 

 sculling tail. It is, however, possible that as in Geosaurus the hind limbs were 

 not typically uatatory and that the distal end of the tail was vertically expanded. 



No specimens have yet been found which are well enough preserved to show 

 any remains of the stomach contents, and we have no delinite evidence concern- 

 ing the food of the Thalattosaurs, more than is furnished by the general structure 

 of the animal. The character of the paddles, the form of the skull, and the 

 presence of slender prehensile teeth in the terminal ])ortions of the jaws would 

 indicate that they fed in part upon some swiftly moving prey which was caught 

 by a quick snap of the jaws, deglutition being assisted by the c'urved teeth of the 

 pterygoid. The heavy vomerine and posterior mandibular teeth may have been 

 used for crushing the light shells of ammonites, which existed in vast numbers in 

 the same seas. 



AFFINITIES AND SYSTEMATIC POSITION. 



As would l)e expected in forms occurring in the Triassic, the Thalattosauria 

 exhibit many features which aic generally considered as j)rimitive. Among 

 these are the biconcave vertebrae, single-headed ribs, w(dl developed abdominal 

 rib plastron, narraw sca[)ula, plate-like coracoid, sculptured jaw elements, strong 

 vomerine and pterygoid dentitions, and the presence of a parietal foramen. It 

 is, however, bj^ no means a primitive type, but is really one of the more specialized 

 of the reptilian groups of the Triassic. Peculiar or specialized characters 

 appear in the structure of the limbs, in the rostral region of the skull, in the 

 position of the narial openings, in the relations of the nasals and premaxillaries, 

 in the character of the parietals, and in the nature of the dentition. 



Owing to the considerable degree of specialization of the particular kind 

 which we find in this group, it would almost necessarily be true that any order 



