1859.] OWEN REPTILIAN FOSSILS. 53 



traversed along its middle by a low ridge : the sides of the pre- 

 maxillary, in the present species, increase a little in depth as they 

 approach the alveolar border. This border, in front of the canine, 

 forms an open angle with the part of the border behind the canine, 

 the one passing into the other with a convex curve on the inner side 

 of the socket of that tooth. 



The rami of the lower jaw augment in depth from the angle to the 

 symphysis, where they meet at an acute angle and are confluent. The 

 angle projects a very little way beyond the articulation ; it is continued 

 inward a short way, and is slightly bent down. The articular surface 

 is moderately concave, and looks obliquely upward and backward. The 

 elements of the posterior half of the ramus, answering to the articular, 

 angular, and surangular in lizards, are too closely compacted together 

 in the specimens under examination to permit an exact definition of 

 their limits. A thin vertical " splenial " plate, on the inner side of 

 the ramus, begins about an inch in advance of the angle, and extends 

 forward to the symphysis, at the back part of which it appears to be- 

 come confluent with its fellow. The part answering to the " angular " 

 diverges from the surangular, and forms the hind boundary of an 

 oblong vacuity at the middle of the side of the ramus, the fore part 

 of which vacuity is formed by a bifurcation of the dentary element : 

 the fore part of the angular piece is continued forward between the 

 lower branch of the dentary and the splenial to the symphysis, where 

 it penetrates a fissure either in the dentary or between the dentary 

 and splenial : it forms the lower boundary of the vacuity at the 

 middle of the ramus. The upper boundary is formed by the upper 

 branch of the dentary, which overlaps the fore part of the surangular. 

 The dentary is thickened and strengthened by a ridge or rounded 

 rising, continued forward from the upper boundary of the fissure, and 

 subsiding at the vertical channel upon the side of the symphysis, 

 receiving the tusks when the mouth is closed. The symphysis of the 

 mandible (fig. 3 32) in both species of Pti/chognathus is peculiarly 

 massive — broad, high, and thick. Anteriorly it is convex in every 

 direction; it is bent or produced upward, terminating in a broad, 

 convex, subtrenchant or trenchant margin, and resembles the Pore 

 part of the lower mandible of a Maccaw. The upward development 

 of tho fore end of the lower jaw is necessitated by the oblique 

 truncation of the pivmaxallary, — the mouth here opening obliquely 

 upward, as in some Fishes, giving a very odd physiognomy to the 

 >kull of Ptychognathus. 



The modification of the hack part of the head of Ptychognaikua, 

 especially the great expansion due exclusively to the development of 

 ridges for augmenting the surface of attachment of muscles ((<>r the 

 brain of the cold-blooded reptile would need but a small spol of the 

 ei 'litre of the occipital plate-, for its protection), indicate-, the power 

 thai was brought to bear upon the head as the framework in which 

 were strongly fixed the two large tusks. The power of resistance of 

 the cavities receiving the deeply implanted bases of the tusks was 

 increased by the ridges developed from the outer part of their bonj 



wall. 



