119 



cavity of the palatine. They are toothless, and have no sutural connection 

 with the ossa quadrala. They present no sutural facet for a columella; but 

 Goldfuss and Marsh believe that the latter element exists. I have not 

 seen it. 



The os quadratum is a stout bone, and one of the most uniformly pre- 

 served among the bones of fossilized individuals. In general, it forms a half- 

 disk ; the convex border thin, one side concave, and the posterior border thick- 

 ened. The proximal end is produced backward beyond the line of this 

 border, forming a hooklike process, which is decurved, nearly inclosing the 

 auricular meatus. Just anterior to the latter, on the inner face of the thick- 

 ened portion, is a pit, which received the end of the stapes or stapedial carti- 

 lage. The vertical plane of the inferior or mandibular condyle is oblique to 

 that of the superior. 



The superior extremity of the os quadratum appears to have had consid- 

 erable motion on the opisthotic. Its extent is so much greater than that of 

 the cotyloid or glenoid cavity, applied to it, as to indicate a gliding motion, 

 especially as it constitutes an extensive arc, possessing grooves of attachment 

 for articular cartilage throughout its length. This arc is bent or curved in 

 the horizontal plane, which would result in a twisting of the os quadratum 

 round its long axis, should the motion I suggest have taken place. Such a 

 twist would throw the proximal portion of the ramus of the jaw outward, a 

 motion quite necessary to the horizontal flexure of the ramus at the splenial 

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

 extent of this outward deflexion of the articular, coronoid, &c, 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 in M. dekayi and 

 M. maximus (see cuts, Fig. 48); the great projection of the external angle in 

 Liodon gave that species an excessive power of dislocation, and the same 

 peculiarity in Clidastes 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 articu- 

 lar surface extends over the internal angle and over the upper edge of the ala, 

 forming the alar process (see plates). Below the meatus and knob, on the 

 postero-external margin, there is a ridge, which terminates 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 - 



