MERRIAM THK THALATTOSAURIA 33 



marine representatives of this ^reat rhynchocephalian or diaptosaurian group. In 

 accommodation to marine conditions their specialization toward tlic natatory- 

 type was carried farther than it would naturally go in forms living in the more 

 narrowly circumscribed bodies of fresli water. In sea forms a continued or sus- 

 tained existence in the water, and even some distance away from the sliore, was 

 probably advantageous. !^uch a tendency would necessarily result in the evo- 

 lution of the only type of extremity that would be serviceable; namely, the 

 |)a(ldl('. In lK)dies of fresh water where the shore is in general easilj- accessible 

 the functions of the limbs would be divided between crawling and swimming. 



Of the seven orders tentatively placed in the Diaptosauria by Osborn none 

 are composed of typical natatory, marine forms. It would liave been remarkable 

 liad tlicre not been developed in tlie adaptive expansion of tliis group just such 

 a type as is represented here. In tlie relation of the Thalattosaurs to tlie other 

 diaptosaurian orders we find a situation similar to that in the i^quamata, the 

 Tiialattosaurs showing the same kind of adaptive specialization away from the 

 primitive Diaptosauria that the Mosasaurs have exhibited with relation to the 

 primitive Lacertilia. 



GENERIC AND SPECIFIC CHARACTERIZATIONS. 



In general, the structure of tlic known specimens indicates close relation- 

 ship of the forms wliich they represent. There is, however, a certain degree of 

 variation, particularly of the dentition and limbs, which is best expressed bv 

 separating them into two or mcjre genera. 



THALATTOSAURUS Mcrriam. 



Bull. Dfpt. (u-ol. Iniv. Calif, v. 3, no. 21, p. 419. 



Type f^pecios. TlidldftdMno-iis aJrxnnd me. 



Posterioi' numdibular teotli, broad-elliptical in cross-section, low-crowned: 

 median teeth increasing in lieiglit anteriorly through the develo})inent of 

 an antero-external tubercle. Anterior mandibular and premaxillarv teeth 

 slender conicah MaxiUary teeth conical in one species, imperfectlv known in 

 the others. Humerus sliort, expan(h'(l distally, witli high pectoral ridge. Radius 

 and ulna much shortened. 



(S) 



