ANATOMY OF THE TOETOISE. 245 



structure belonging to the epidermis. It is in the form of thin plates, which are united together at 

 their edges, and which correspond, to a certain extent only, with the underlying bones of the shell. 

 The number, size, and position of these plates differ in all the great groups which possess them, 

 and even in the genera and species, and so do their colours and ornamentation. In most there 

 is a very geometrical configuration to the plates and to their grouping: and as large pieces of the 

 epidermoid covering, or " tortoiseshell," are the best, those of the Hawk's-bill Turtle are the most in 

 request. Usually, the position of the tortoiseshell plates is impressed on the carapace and plastron 

 beneath them, the edges being recognised as indentations. 



The breast-bone, or sternum, which is so well seen in Birds and Mammals, does not exist in the 

 Chelonia, neither are there any 

 sternal ribs ; moreover, the ver- 

 tebrse which enter into the com- 

 position of the carapace have no 

 transverse processes. Only the 

 ribs are present at the sides, and 

 they arise between the body of 

 the vertebrae and their neural 

 arch, which is capped by the 

 expanded spinous process. SKELETON OF TORTOISE, FROM ONE SIDE. 



Consolidated and compara- 

 tively motionless as are the bones and plates of the carapace and plastron, this is not the character 

 of the rest of the skeleton, for the neck, tail, and limbs can be active enough, and their anatomy is in 

 accordance with their mobility. 



A remarkable little Tortoise* has a long plastron, which can be bent across just behind the first 

 quarter of its length, and it is broad and very close behind. There are openings for the head and 

 neck, and for the four limbs, so that the armour is very complete. When the animal is alarmed, aiirl 

 withdraws its head and fore-limbs under the carapace, the movable front part of the plastron 

 closes over their openings and effectually covers them ; and when it desires to move again, the traj>- 

 door is opened by being lowered, and the head and fore-limbs can come forth. Another Tortoise,f 

 belonging to the same great groxip of Land Tortoises as that just mentioned, has the carapace 

 movable behind, but not the plastron in front, and the animal protects its tail and hind-limbs 

 effectually by bending it down. But this protection does not take place through the agency of 

 an elastic ligament, as in the Pyxis Tortoise, allowing the plastron to be bent. On the contrary, the 

 plastron does not move, but the vertebrae, ribs, and plates of the hinder part of the carapace 

 are not rigid, and can be bent down and curved by muscular action, so as to shut up the posterior 

 openings, by approaching the back surface of the plastron. This last is rounded behind, and in its 

 general construction it differs somewhat from that of these so-callecl Box Tortoises. One of the most 

 perfectly protected Chelonians. although it belongs to a soft-skinned group called Mud or Soft 

 Tortoises, has three boles in the back part of the iinited buckler. Amongst the Chelonians with 

 incompletely protecting bucklers are the Turtles, for vast as their shields may be, still their head and 

 limbs cannot be brought within them as in the Tortoises. Still greater want of covering is noticed 

 in the Alligator Terrapin J of North America, and in the great-headed Platysternon of China. 



The feet are not heeled in the Chelonia, so that they have a hand-like look, and the ankle-joint 

 beneath the tibia and fibula has only one bone, in contact with it in some kinds, and two in others. 

 Four other bones are present in the Tortoise, which has five small toes, the little toe projecting from 

 a metatarsal bone that sticks out from the foot. There is a good amount of fleshy padding under 

 the fingers and toes in some kinds, and a web exists between them in others. The swollen 

 appearance of the under part of the feet of the Tortoise is remarked by every young naturalist 

 who has kept them as pets, and the nails are attached to the very short digits which seem like 

 little excrescences. But other Chelonians, such as the marine and the fresh-water kinds, have 

 well-developed fingers and toes, with webs and nails. The Turtles especially move with great 

 grace, the hands and feet taking 011 a " feathering " motion, like paddles. 



* Pyxis arachnoides. t Kinixys homeana. J Chelydra serpentina. Plati/sternon megacephdlum. 



174 



