68 



Shielded Keptiles- 



-KEPTILES.- 



-Land Toktoises. 



its two extremities, the head and tail. The plastron 

 is equivalent to the breast-bone or stermim in birds 

 and mammals, and is composed of nine jiieoes, which 

 in general are firmly united together — fig. 21. It covers 

 all the lower part of the belly of the animal, is solid 

 in the land and most of the fresh-water species, but 

 varies very much in form and structure. Round the 

 edge of the carapace there are a series of bones joined 

 together, which represent the cartilages which join the 

 ribs to the sternum in birds, &c. From this structure 



SterDuin of Chclullia. 



there results, of course, a complete immovability in 

 all these parts, and an immense degree of strength. 

 Their head is generally rather small, and of a flattened 

 form, and the jaws are covered with a hard, sharp, 

 horny beak, the lower portion of which shuts within 

 the upper. This structure supplies the place of teeth, 

 and the portions of food are cut or snapped off on the 

 principle of shears. Though they do not bite very 

 readily, yet, when they do, they bite very severely. 

 There is no possible means of making them let go 

 their hold. Even killing them will not always suffice. 

 They will retain the piece unless the jaws be completely 

 broken. So forcible, indeed, and so violent is their 

 bite, " that I have known," says Mr. Bell, " a stick of 

 half an inch in diameter at once snapped asunder by 

 the jaws of a Snapping Turtle ; and a specimen of 

 Trionyx, lately in the possession of Mr. Cross of the 

 Surrey Zoological Gardens, snapped off the finger of a 

 sailor when on his voyage to this country." The 

 limbs of tortoises and turtles present a great contrast 

 to that of most other vcrtebrated animals. They are 

 short and thick, and in all are far removed from the 

 centre of gravity. The form and structure of the feet 

 differ much in the different groups, according to their 

 habits of life. In the Land tortoises they are too short 

 to be able to sustain for a long time the weight of the 

 body, or to elevate it above the level of the ground 

 sufficiently to make locomotion easy. They are clumsy 

 and club-shaped, and these animals therefore only drag 



themselves slowdy along the ground, their plastron or 

 breast-plate almost pressing upon it ; and as they move 

 along, their walk is tottering, uncertain, and extremely 

 slow. The marine species, or Turtles, and those which 

 live in rivers, have their feet changed into regular 

 paddles or oars, and endowed with great muscular 

 power. They are thus able to swim well, and their 

 motion through the liquid element, in which they live, 

 is rapid and not without grace. Those species which 

 live in ponds are intermediate between the other two 

 groups, and have the toes webbed and the claws sharp. 

 They are thus able to move along both in the water 

 and on the land ; whilst the Land tortoises would perish 

 in the water were they to live in its immediate neigh- 

 bourhood, and are consequently only found on dry 

 land. The neck in tortoises and turtles is generally 

 cylindrical and capable of great extension, and though 

 the skin is almost always covered with small scales, 

 which are separate and hard, it is the most defenceless 

 part of the animal, and that in which it may most 

 easily be killed. The legs and feet are generally 

 covered with scales also, which serve as a considerable 

 protection to them from accidents or injury ; and the 

 texture of this skin is so solid, that the sharpest instru- 

 ments can with difficulty penetrate it. The senses 

 possessed by these creatures are in general for from 

 being acute. The range of their sensations may be 

 said to be confined to the strictest limits of necessity, 

 or, in other words, merely to what is indispensable for 

 the purposes of self-preservation and reproduction. 



LAND TORTOISES {Testudinida). 



The land species are all comprised in one family, 

 Tegtudiuidce, or Tortoises properly so-called ; and of 

 all the Chelonia are those which have their shell 

 composed of the thickest and heaviest pieces. In 

 the adult state it is covered with horny, concen- 

 trically-grooved shields, which are marked with a 

 permanent areola ; and thus the shell is never smooth 

 on the surface, as is the case with marine turtles, 

 &c. The bead is in nearly all the species pro- 

 portionally of the same size ; in geneial it is short, 

 thick, and quadrangular. The eyes are placed laterally 

 and on the same level with the head. The eyelids are 

 cleft obliquely in such a way that the anterior angle is 

 nearly of the same height as the nostrils, whilst the 

 hinder angle is a little higher. The nostrils open at 

 the extremity of the muzzle. The tongue is thick and 

 covered on the upper part with papillcE. The horny 

 sheaths which cover the bones of the jaws are very 

 solid and sharp-cutting, or, in some, more or less 

 toothed ; and the jaws themselves close like the lid of 

 a box, and can only act in one way, like the blades of 

 scissors. The head and neck can always be withdrawn 

 within the shell. But the most distinctive charactei 

 of the Land tortoises resides in the structure of the feet. 

 The hinder and fore legs are nearly or quite of the 

 same length; the foot is hard, and of a truncated stump 

 form, like that of tlie elephant. When they walk, or 

 as it were drag themselves slowly along, this foot is 

 turned obliquely outwards, for it never rests completely 

 ou the ground, the claws in fact forming the sup- 



