362 PROFESSOR OWEN, ON THE RHYNCHOSAURUS. 



this part prevents the determination of the precise form of this aperture, 

 or the mode of termination of the nasal bones. These bones, if not 

 actually absent in the present fossil, as in most Chelonia, must have 

 been extremely small, as in the Chameleon. 



The lower jaw is of considerable depth, and exceeds, as in most 

 Saurians, the length of the cranium. The articular cavity is deep and 

 wide: the angle of the jaw is broken off in the fossil directly behind 

 this cavity on the left side, but is continued backwards beyond it 

 for more than half an inch on the right side. The ramus gradually 

 expands in the vertical direction, and becomes thinner from side to 

 side, as it advances forwards, to about its middle part, which is just 

 behind the orbit, where it measures 11 lines in depth : it then be- 

 gins gradually to diminish vertically to the symphysis, which again 

 slightly increases to its termination, which is obliquely truncated, much 

 compressed laterally, and applied against the deflected extremities of 

 the intermaxillaries. The posterior half of the maxillary ramus is slightly 

 convex externally ; the anterior narrower part slightly concave : the 

 superior margin describes a slight but graceful sigmoid curve, convex 

 posteriorly, and concave anteriorly, where it is adapted to the convex 

 alveolar border of the upper maxillary bone, to the inner side of which 

 it is closely applied. The alveolar border forms an external, convex, 

 projecting ridge, analogous to that of the upper jaw. 



The composite structure of the lower jaw is very clearly displayed 

 in the fossil. The articular piece is short, but is continued forwards 

 as a slender process below the angular piece e, as in the Varanus. The an- 

 gular piece is relatively larger than in Varanus, and presents nearly 

 the same proportions as in Thorictes. The supra-angular is longer, and 

 occupies the proportion of the jaw formed by the supra-angular and 

 coronoid elements in Thorictes and other Lizards. The opercular element 

 extends farther upon the outside of the jaw from its lower margin 

 than in the existing Lizards; Thorictes, again, in this respect, coming 

 nearest to Rhynchosaurus. The dentary element constitutes the rest of 

 the outer side of the ramus, but not the slightest trace of teeth is 

 discernible. The fossil seems to have been preserved with the mouth 



