AND AVES OF NORTH AMERICA. 



183 



MOSASAUKID.K. 

 Dentition, In this family the forms of the teeth differ in different parts of the maxil- 

 lary bones. Those on the premaxillaries are more cylindric in section, and the compres- 

 sion, or development of angles or facets, whichever it be, increases as they succeed each 

 other posteriorly. One peculiarity characterizes the teeth of the most of the true Mosa- 

 sauri, viz: the anterior aspect bears a, cutting edge more or less developed, while in Lio- 

 don the posterior is more strongly developed. 



Os Quadratum. The superior extremity of the os quadratum appears to have had 

 considerable motion on the opisthotic. Its extent is so much greater than that of the co- 

 tyloid or glenoid cavity applied to it, as to suggest a gliding motion, especially as it con- 

 stitutes an extensive arc, possessing grooves of attachment for articular cartilage throus-h- 

 out its length. This arc is, however, bent or curved in the horizontal plane, which would 

 result in a. twisting of the os quadratum round its long axis, should the motion 1 su<vo- e st 

 have taken place. Such a. twist would throw the proximal portion of the ramus of the 

 jaw outwards, a, motion quite necessary to tin? horizontal flexure of the- ramus at the 

 splenial articulation, which no doubt took place in swallowing any large object. The ex- 

 tent of this outward deflexion of the articular, coronoid, etc., portions of the jaw, was 

 measured by the outward concavity of the proximal end of the quadratum. Thus this is 

 least in M. depressus, and greater iii M. dekayi and M. maximus (see cuts fig, 48); the 

 great projection of the external angle in Liodon validus, gave that species an excessive 

 power of dislocation, and the same peculiarity in Clidastes propython, was followed by 

 the same effect. 



As the development of processes and ridges on the ossa quadrata differ in the different 

 species, they may be named as follows: The proximal articular surface extends over the 

 external angle, and over the upper edge of the ala forming the alar process (see cuts). 

 Below the meatus and knob, on the postero-oxternal margin, there is a, ridge which ter- 

 minates in a, process in some species, to be called the median posterior ridge. In some a. 

 ridge rises from the outer angle of the distal articular face, extending outside- the ridge 

 just mentioned, towards the pit, called the distal external longitudinal. In front of this. 

 on the outer face of the quadrate behind or near the origin of the ala, may he a ridge 

 called flu; external ridge. See Tab. XI, figs. (i-7. 



Vertebras. As has been already pointed out by Cuvier, the vertebra in this genus fall 

 into cervical, dorsal, sacro-himbar and caudal series. The cervicals are either round or 

 depressed in this genus; in Liodon the articular faces are either round or compressed in 

 the species I have examined. They are arbitrarily characterized by the presence of an 

 obtuse hypapophysis which has an articular surface for a separate continuation of the 

 same, as in Clidastes. None of these pieces have come under my observation in this 



