262 FOSSIL REPTILES. 



form a large right angle above, unite by their descending parts 

 to the palatines, the pterygoideans, the ossa petrosa, and the 

 upper occipitals. They form of themselves almost the entire 

 penthouse of the cranium. At the sequel of the pterygoidean 

 the temple is bounded behind by the tympanic bone, which 

 partly resembles a trumpet. The frame of the tympanum is 

 complete. A foramen of the hinder paries lets the osselet 

 pass into the second chamber, which, in the skeleton, is only a 

 long groove of the posterior face of the os tympani, which ter- 

 minates in a cavity, to the formation of which the os petrosum 

 and the external and lateral occipitals concur. It is closed 

 behind only by cartilage and membranes. The fenestrse are in 

 their usual place. 



Above this foramen of the first chamber, through which the 

 osselet passes, is another which conducts into the mastoidean 

 cell, which, in consequence of the outward projection of the 

 tympanum, is found within, and not behind. The occipital 

 spine is a short vertebral crest, and the mastoid tubercles are 

 transverse crests belonging entirely to the mastoidean bone. 



Underneath, the cranium is smooth and almost plane, and 

 exhibits a sort of regular compartment, formed of the inter- 

 maxillaries, the maxillaries, the vomer, the palatines, the ptery- 

 goideans, the sphenoid, the ossa petrosa, the ossa tympani, the 

 basilary, and the lateral and external occipital bones. Behind 

 the floor of the temple the os petrosum forms a square com- 

 partment, between the pterygoid, the tympanum, the external 

 occipital, the upper occipital, and the parietal. 



In the lower jaw, the space occupied in the crocodile by the 

 two dentary and two opercular bones, is occupied in the marine, 

 fresh-water and land-tortoises by a single bone analogous to the 

 two dentaries. There is no trace of symphysis, the bone being 

 continued as in birds. But in the testudo fimhriata a division 

 is preserved at the anterior part, at every age. The opercular, 

 however, exists at the internal face of the jaw, but is thrown 

 more backward than in the crocodile. Under it is the angular, 



