SHIELDED REPTILES. 



TORTOISES. 



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'HE 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 that when the animal has withdrawn its limbs and head, it is contained in a tightly closed 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 cara- 

 pace, and the lower the plastron. 



The carapace is formed by a remarkable development of the vertebra? and ribs, which 

 throw out flat processes, and are joined together by sutures like the bones of the skull. The 

 back is therefore incapable of movement, and from the arched shape of the bones is wonder- 

 fully 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 breast-bone, 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. 



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 vertebrate animals, the 

 portion which represents the true shoulder-blade being almost cylindrical, one of the branches 

 flattened, and the other cylindrical, but larger than the real blade-bone. This structure admits 

 of the attachment of powerful muscles, and gives to the fore limbs the great strength which is 

 needed for digging, swimming, climbing, and various modes of exertion. The strong curved 

 bones of the fore limbs bear an evident analogy to the corresponding parts in the mole, with 

 its powerful claws and feet, and its very long blade-bone. 



The horny substance commonly termed "tortoise-shell," which is spread in flattened 

 plates on the exterior of the bony case, is thought to be a modification of the scales found on 

 lizards, serpents, etc., and which exist on the legs and other parts of the Tortoises themselves. 

 The row of horny pieces which are found on the edge of the carapace also belong to the 

 ' ' dermal skeleton. ' ' 



The Tortoises are quite devoid of teeth, the edge of the jaws being sharp and horny, so as 

 to inflict a severe wound ; and in many species one or both jaws are sharply hooked at the tip 

 like a falcon's beak. The neck is always rather long, and in many species can be protruded 

 to a considerable extent. Generally, the process of thrusting the neck from the shell is a slow 

 one, but the withdrawal is accomplished with marvellous rapidity, on account of certain long- 

 muscles which tie the neck to the back of the carapace. Possibly these muscles, together with 

 their tendons, would, when dried in the baking sunshine, produce musical sounds when 

 touched, and thus give rise to the old poetical legend of the origin of the lyre 



