CHELONIA. 5 



of the ribs, and which in one subgenus always remains in a 

 cartilaginous state, surrounds the shell, uniting and binding to- 

 gether all the ribs which compose it. The vertebrae of the 

 neck and tail are consequently the only ones which are mov- 

 able. 



These two bony envelopes being immediately covered by 

 the skin or by plates, the scapulae and all the muscles of the 

 arm and neck, instead of being connected with the ribs and 

 spine, as in other animals, are attached beneath : the same ar- 

 rangement is found in the bones of the pelvis and all the mus- 

 cles of the thigh, so that in this respect the Tortoise may be 

 said to be an inverted a-nimal. 



The vertebral extremity of the scapula is articulated with 

 the shell ; and the opposite limit, which may be considered 

 analogous to a clavicle, is joined to the sternum. So that the 

 two shoulders form a ring through which pass the oesophagus 

 and trachea. 



A third bony branch, larger than either of the others, and 

 directed downwards and backwards, represents, as in Birds, 

 the coracoid apophysis, but its posterior extremity is free. 



The lungs have considerable extent, and are situated in the 

 same cavity with the other viscera. (1 ) The thorax, in most 

 of them, being immovable, it is by the play of its mouth that 

 the Tortoise respires, which it effects by keeping the jaws 

 closed, and alternately raising and depressing the os hyoides. 

 The former of these motions permits air to enter through the 

 nostrils, the tongue then closes the internal orifice of those 

 apertures, when the latter forces the air into the lungs. (2) 



Tortoises have no teeth ; their jaws are invested with horn 

 like those of Birds; the Chelydes excepted, where they are 

 covered with skin only. Their tympanum and palatine arches 



(1) observe that in all those reptiles in which the lung penetrates into the ab- 

 domen (and the Crocodile is the only one in which it docs not) it is enveloped like 

 the intestines by a fold of the peritoneum, which separates it from the abdomi- 

 nal cavity. 



(2) With respect to this mechanism, which is common to Tortoises and to the 

 Batrachianp, see the Mem. of llobert Townson, Lond. 1779. 



