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 when the animal has withdrawn 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 

 remarkable development of the 

 vertebras and ribs, which throw 

 out flat processes, and are joined 

 together by sutures like the bones 

 of the skull. The back is there- 

 fore 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 mentioned. These bones 

 are arranged in four pairs, and 

 one in the centre of the front. 



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

 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 TORTOISi:. 



As 

 organs 



