SKELETON OF TORTOISE. 



TORTOISES. 



THE very curious reptiles which are known by the general name of Tortoises, are 

 remarkable for affording the first example of a skeleton brought to the exterior of the body, 

 a formation which is frequent enough in the lower orders, the crustaceans and insects 



being familiar examples thereof. 

 In these reptiles the bones of the 

 chest are developed into a curious 

 kind of box, more or less perfect, 

 which contains within itself all the 

 muscles and the viscera, and in 

 most cases can receive into its 

 cavity the head, neck, and limbs ; 

 in one genus so effectually, that 

 when the animal has withdrawn its 

 limbs and head, it Is contained in a 

 tightly closed case without any 

 apparent opening. 



The shell of the Tortoise is divided 

 into two portions, the upper being 

 termed the carapace, and the lower 

 the plastron. 



The carapace is formed by a re- 

 markable development of the ver- 

 tebrae and ribs, which throw out 

 flat processes, and are joined to- 

 gether by sutures like the bones 

 of the skull. The back is therefore 

 incapable of movement, and from 

 the arched shape of the bones is 

 wonderfully strong when resting on 

 the ground. In the Tortoises these 

 bones are united throughout their 

 entire length, but in the Turtles the 

 ends of the ribs retain their original 

 width. 



The plastron is similarly formed 

 of the breastbone, which is thought 

 in these creatures to be developed 

 to the greatest extent of which it is 

 capable. It is composed of nine 

 pieces, each being formed from one 

 of the bony centres already men- 

 tioned. These bones are arranged in four pairs, and one in the centre of the front. 

 As all the limbs have to be worked from the interior of the chest, amid the vital 

 organs and muscles for moving them, they undergo considerable modification. The 

 shoulder-blade, for example, is a curious three-branched bone, quite unique among 



SKELETON OF TORTOISE. 



