CLASS OF EEPTILES. 



315 



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. 273). 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 homy 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 ske- 

 leton in the other vertebrata merely changed 

 in form and volume. 



When we examine the carapace superiorly, 

 we find it composed of a great number of 

 osseous plates, united by sutures, of which 

 eight occupy the median line ; sixteen others 

 form on each side a longitudinal row; and 

 twenty-five or twenty-six surround the whole. 

 We have only to look at the inner side of 

 the carapace (Fig. 274) 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 mar- 

 row may be 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 to the preceding 

 and following vertebrae. The dorsal vertebrae 

 become thus immovable : each carries a pair 

 of ribs as in man, but these (<?) 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 (cs) which articulate with the extremity of the 



Fig. 272. 

 Chacide. 



