FOSSIL REPTILES. 263 



constituting the lower edge of the jaw, and the external face of 

 this part is occupied by the subangular, which touches not the 

 angular except very far back, being separated from it on the 

 two anterior thirds of its length by the dentary. 



Above and towards the back, between the opercular and 

 subangular, is situated the articulary bone. Its dimensions 

 are but small, and it serves only for the articulation and inser- 

 tion of the depressor muscle, which is analogous to the digas- 

 tric. 



The coronoid apophysis does not belong to the subangular 

 bone, but to a bone placed between the dentary, opercular, 

 and subangular, and in front of the aperture by which the 

 nerves enter the jaw, which aperture is here on the upper edge, 

 instead of being, as in crocodiles and birds, at the internal 

 face. 



The hyoid bone is very complicated and singularly varied in 

 conformation in the genera and species of tortoises, but it is not 

 necessary for our purposes to enter into a detailed description 

 of it. 



From what has preceded, it may be observed that there is 

 more difference in the arrangement and mutual relations of 

 the bones of the head in the different tortoises, than pro- 

 bably in the heads of all quadrupeds, and most assuredly 

 than in the entire class of birds. There are proportional, 

 though not so considerable, differences in the other parts of the 

 skeleton. 



The most general character of the tortoises, as is well 

 known, consists in their having the bones of the thorax out- 

 side, enveloping with a cuirass or double buckler what subsists 

 of the muscles, and serving even as a shelter to the bones of the 

 shoulder and the pelvis. This dorsal buckler is formed prin- 

 cipally of eight pairs of ribs, united towards the middle by a 

 longitudinal series of angular plates, which either adhere to the 

 annular parts of so many vertebrae, or constitute a part of 

 them. What is most remarkable is, that these annular parts 



