ZOOLOGY. 



through the lungs, the greater part passing again 

 into the circulation without being aerated. Hence 

 it is that the blood is cold, the digestive powers 

 weak, and their vitality low, so that their functions 

 may be suspended for a considerable time without 

 apparent injury to the animal. The skull is 

 articulated to the vertebral column by a single 

 occipital condyle. 



ORDER i. Chelonia. In typical species, the 

 ribs are expanded so as to form but one bony 

 plate, having no flesh outside, but being covered 

 with horny plates, secreted from the skin like hair 

 or nails ; the breast-bone is similarly expanded 

 into a plate, covering the whole of the lower sur- 

 face, and joining the edges of the upper plate ; so 

 that the animal may be said to be sheltered in a 

 box formed of its bones. Within this box, the 

 upper plate of which is named the carapace, and 

 the lower the plastron, the animal can even with- 

 draw its head and feet, and thus set most enemies 

 at defiance. There are no teeth, and the edges of 

 the jaws are sheathed in horn, so as to form a 

 kind of beak. 



The family Chelonidcs (Turtles) are designed 

 for sea-life, and their extremities take the form of 

 paddles. They swim with great ease, and only 

 come to land to deposit their eggs, which they 

 do thrice a year, laying about 100 at a time. They 

 feed chiefly on marine plants. They are sometimes 

 five feet in length, and weigh 800 pounds. The 

 Chelone midas (Edible or Green Turtle) of the 

 tropical seas of America, is noted for the delicious 

 food which it yields. Ascension Island is one of 

 their favourite retreats. 



The Chelone imbricata (HawkVbill Turtle) is a 



I, Hawk's-bill Turtle (Chelone imbricata}; 2, Green 

 Turtle (CMone midas). 



smaller species, and is remarkable for the imbri- 

 cate beautifully marked horny plates covering its 

 carapace, which yields the tortoise-shell of com- 

 merce. These plates do not join at the edges like 

 those of land-tortoises, but overlap each other like 

 the scales of other reptiles. The finest tortoise- 

 shell is brought from the Indian Archipelago. 



The Chelone or Sphargis caretta (Leathery 

 Turtle) has the carapace covered by a leathery 

 skin in place of horny plates. This animal in- 



habits the Mediterranean as well as the Atlantic 

 and Pacific Oceans. 



The family of the Emydce (Terrapenes) and the 

 TrionytidcB (Mud Turtles) have the feet furnished 

 with toes, and being partly webbed, they are useful 

 for swimming. The Snapping Turtle (Trionyx 

 ferox) of America is capable of biting through a 

 stick half an inch in diameter. The T. Niloticus is 

 highly serviceable in the Nile and other rivers in 

 destroying young crocodiles and alligators. They 

 are all inhabitants of fresh water. 



The Testudinidce (Land Tortoises) have the feet 

 furnished with short nails, and adapted for walking 

 on firm ground. Their armour is thicker and 



Common Land Tortoise ( Testuda graca). 



stronger in proportion to their size than the aquatic 

 species. They are vegetable feeders. The T. 

 grceca of Spain is often kept as a domestic pet. 



ORDER 2. Ophidia have an elongated cylin- 

 drical body, covered with scales, but destitute 

 of limbs, though in some serpents rudimentary 

 posterior limbs can be traced, but these are of 

 no use for progression. The ribs are exceedingly 

 numerous, and very movable, and their pointed 

 extremities are attached by muscles to the ab- 

 dominal scales, or 'scuta,' of the integument. 

 By this arrangement, the animal moves along on 

 the points of its ribs, the whole being facilitated 

 by the very movable spinal column. The tongue 

 is forked, and there is no eyelid, the eye being 

 covered with a glassy capsule, within which the 

 eye moves freely. All are furnished with teeth, 

 which never occupy distinct sockets, but are 

 united to the jaw, and are arranged either in two 

 or three rows. The two halves of the lower jaw, 

 which are composed of several pieces, are united 

 in front by ligaments and muscles, which permits 

 of their being separated to a considerable distance, 

 so that, though the head is small, these animals 

 are enabled to swallow their prey entire. They 

 remain torpid during winter, and each season cast 

 their skin. 



Sub-order i. Viperina, or Venomous Snakes 

 have a pair of perforated poison-fangs in the 

 upper jaw, which can be erected or depressed at 

 will. When the animal is irritated, they are un- 

 folded, and struck into the victim, the poison 

 being at the same time^jected by the action of the 

 muscles of the jaws. The matter is poisonous only 

 when introduced into the blood. The Viperina 

 comprise the Rattlesnakes (Crotalidce) of America, 

 which are distinguished by a pit behind each 

 nostril, and by the presence of the rattle. They 

 are highly venomous, but they will not attack man 

 unless when trodden on or provoked. The ex- 

 tremity of the tail in Crotalus horridus is furnished 



163 



