260 PALEONTOLOGY. 



dentary is thickened and strengthened by a ridge, continued 

 forward from the upper boundary of the fissure, and subsiding 

 at the vertical channel upon the side of the symphysis, receiv- 

 ing the tusks when the mouth is closed. The symphysis 

 of the mandible (3 a, 32) 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, 

 trenchant margin, like the fore part of the lower mandible of 

 a maccaw. The upward development of the fore end of the 

 lower jaw is necessitated by the oblique truncation of the pre- 

 maxillary — the mouth here opening obliquely upward, as in 

 some fishes, giving a very odd physiognomy to the skull of 

 Ptychognathus. 



The modification of the back part of the head of Ptycho- 

 gnathus, especially the great expansion due exclusively to the 

 development of ridges for augmenting the surface of attach- 

 ment of muscles (for the brain of the cold-blooded reptile 

 would need but a small spot of the centre of the occipital 

 plates for its protection), indicates the power that 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 bony wall. 



Only the crocodiles now shew a like extent of ossification 

 of the occiput, and only the Chelonians the trenchant tooth- 

 less mandible : but in. both the outer nostril is single and 

 median ; the lizards repeat the divided apertures for respir- 

 ing air : in mammals alone do we find a development of 

 canine tusks like that in the Dicynodonts. 



Ptychognathus latirostris, Ow. — A second species of 

 Ptychognathus is indicated by a skull which in its 

 facial part is broader and shorter, and which has the orbits 

 of a more circular form, yet presenting the notch at the 



