4 REPTILES. 



pace, is formed by the ribs, of which there are eight pair, widened and 

 reunited by denticulated sutures, and with plates adhering to the annular 

 portion of the dorsal vertebrae, so that all these parts are rendered fixed 

 and immoveable. The inferior shell, called plastron, is formed of pieces, 

 usually nine in number, analogous to a sternum*. A frame, composed of 

 bony pieces, which have been considered as possessing some analogy with 

 the sternal or cartilaginous portion of the ribs, and which in one subgenus 

 always remains in a cartilaginous state, surrounds the shell, uniting and 

 binding together all the ribs which compose it. The vertebrae of the neck 

 and tail are consequently the only ones which are moveable. 



These two bony envelopes being immediately covered by the skin or by 

 plates, the scapulse and all the muscles of the arm and neck, instead of 

 being connected with the ribs and spine, as in other animals, are attachcil 

 beneath: the same arrangement is found in the bones of the pelvis and 

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

 Kaid to be an inverted animal. 



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 f. The thorax, in most of them, being im- 

 moveable, it is by the play of its mouth that the Tortoise respires, the 

 process being effected 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 [J. 



Tortoises have no teeth; their jaws are invested with horn, like those 

 of Birds, — the Chelydes, where they are covered with skin only, excepted. 

 Their tympanum and palatine arches are fixed to the cranium, and are 

 immoveable ; their tongue is short and bristled with fleshy filaments ; their 



* See Geoff. An. du Mus. t. XIV, p. 5; and on the entire osteology of the tor- 

 toises, my Rech. sur les Oss. Foss. torn. V, 2e partie («). 



t Observe that in all those reptiles in which the lung penetrates into the abdo- 

 men (and the Crocodile is the only one in which it does not), it is enveloped like the 

 intestines by a fold of the peritoneum, which separates it from the abdominal cavity. 



X With respect to this mechanism, which is common to Tortoises and to the Ba- 

 trachians, see the Mem. of Robert Townson, Lond. 1779. 



{^" (a) In future, the upper shell will be called " shell," and the inferior shell, 

 sternum." — Eng. Ed. 



