350 ZOOLOGY. 



leuvre (adder or snake) there are more than three hundred 

 pairs of ribs. The vertebrae also, by their mode of articula- 

 tion with each other (a kind of ball-and-socket), permit of ex- 

 tensive movements. But the most remarkable 

 modification of the skeleton is in the tortoise 

 and turtle ; the bones form a buckler, within 

 which the animal may withdraw itself on the 

 approach of danger. The dorsal osseous plate 

 is called the carapace ; the ventral, plastron 

 (Fig. 348). United at the sides, they leave 

 in front and behind openings for the head and 

 limbs. This kind of cuirass is only covered 

 by the skin, which in its turn is protected by 

 large horny plates : all the muscles and other 

 soft parts are contained within the cavity thus 

 / formed. 



458. Notwithstanding the profound 

 ' modifications which the skeleton undergoes 



to meet such an organization, we find the same 

 elements or pieces which constitute the skele- 

 ton in the other vertebrata merely changed in 

 form and volume. 



When we examine the carapace (dorsal 

 plate) superiorly, we find it composed of a 

 i great number of osseous plates, united by 



sutures, of which eight occupy the median 

 line ; sixteen others form on each side a longi- 

 tudinal row; and twenty-five or twenty-six 

 surround the whole. We have only to look 

 at the inner side of the carapace (Fig. 349) 

 to see that these median pieces are simply 

 dependencies of the dorsal vertebrae (vd). The 

 body of each of these bones and the canal for 

 the spinal marrow may fee seen, with their 

 ordinary shape, but the upper portion of 

 the osseous ring has been spread out like a 

 disc, uniting with the same parts belonging 

 Fig. 347. Chal- ^ o fo e preceding and following vertebrae. 



cis : an elon- __, , r , . & , , . , p . 



gated Lizard. The dorsal vertebrae become thus immov- 

 able : each carries a pair of ribs as in 

 man, but these (c) spread out so as to touch each other 

 throughout almost their whole length, and to articulate with 

 each other by means of sutures ; finally, the marginal pieces 



