AND AVES OF NORTH AMERICA. 



177 



walls, composed of the prolonged prootic in front, and the exoccipital behind, which em- 

 braces the suspensorium for much of its length. 



Eighteenth. The anterior limbs are fins, with all the elements in a single plane ; the 

 radius incapable of rotation ; the humerus broad and flat. 



Nineteenth. There are probably no hind limbs. 



Of the above characters the first eight arc those of serpents; the five characters follow- 

 ing the ninth axe lacertian, while the seventeenth is peculiar, and not found in an\ 

 existing order of reptiles. The eighteenth is characteristic of the Sauropterygia. 



The characters of the teeth are much those of serpents, and much more different from 

 those of any Saurians. As without true dentinal fangs, they are serpent-like ; lor the ossi- 

 fication of the pulp, which produces a fang-dike support to the crown, is not more than a. 

 subordinate character, like that of ossification or non-ossification of cartilages in many 

 existing orders. The pterygoids, which are in contact medially in Mosasaurus, are largely 

 free in Liodon and in Clidastes ; in the latter they bear teeth as abundantly as do 

 many serpents. Among the Lacertilia the dentition is either truly rhizodont (the Acro- 

 donta) or Pleurodont. The teeth of the Varanidae are especially different from those of 

 the present order, and present only a modification of the pleurodont character. The outer 

 parapet of (lie jaw is low, and the shanks proportionately short ; they are in addition mere 

 expanded than in most other pleurodont families. 



The characters preserved by the temporal region are highly peculiar and important in 

 determining the affinities of the group. The discovery of its structure furnishes the de- 

 sired explanation of sundry enigmatical bones which occur not tmfrequently in our Creta- 

 ceous formations. In the following diagnosis the present is compared with the three orders 

 to which it makes nearest approach. 



'Teatudinata. 



Opisthotic distinct, closely united with exoccipital squamosal and prootic, and sup- 

 porting squamosal and quadratum. 



Lacertilia. 



Opisthotic distinct, closely attached to parietal arc, and at extremity to exoccipital 

 and prootic ; supporting squamosal and quadratum. 



/ '//thonomorpha. 



Opisthotic distinct, not or scarcely in contact with parietal arc ; embraced at one end 

 by prootic and exoccipital, and supporting squamosa] and quadratum. 



OpMdia. 



Opisthotic distinct, attached only to prootic, and supporting only quadratum. 



There can be no doubt that the suspensorium of Mosasaurus is homologous with the 

 element in the tortoises called by Huxley opisthotic. It appears, also, to be homologous 



AMERICA. PHILO. 800. — VOL. XIV. 45. 



