452 GENERAL ZOOLOGY 



by movements of the ribs, which are attached at their lower 

 ends to the abdominal scales or scutes. Snakes are either 

 oviparous or viviparous. 



Examples: Coluber constrictor, the common blacksnake; 

 Thamnophis sirtalis, the common garter snake; Agkistrodon 

 mokasen, the copperhead, poisonous; Crotalus horridus, the 

 common rattlesnake, poisonous. 



Order 4. Testudinata. Turtles. The body is enclosed in a 

 dorsal carapace and a ventral plastron, each of which is usually 

 composed of epidermal plates, internally supported by bone. 

 Toothless jaws are covered with horny sheaths. The eyes have 

 upper and lower lids and a nictitating membrane. The 

 skin is usually covered with scales. The limbs are pentadactyl 

 with clawed toes. In marine turtles the limbs are modified into 

 flippers with reduced numbers of digits. Eggs of both aquatic 

 and terrestrial forms are deposited on land. 



Examples: Chelydra serpentina, the snapping turtle; 

 Terrapene Carolina, the box turtle; Malaclemys centrata, the 

 diamond-back terrapin; Chrysemys picta, the painted turtle; 

 Amy da spinifera, the soft-shelled turtle, lacks horny plates or 

 scales and has fleshy lips; Eretmochelys imbricata, the tortoise- 

 shell turtle, marine; Caretta caretta, the loggerhead turtle, 

 is also marine. 



CLASS VI. AVES. Birds. A modern bird is a homoiother- 

 mous animal, whose body is covered with feathers and scales, 

 whose anterior limbs are modified for flight, and whose jaws 

 lack teeth. The heart has two ventricles and two atria and the 

 circulation is completely double. The stomach consists of two 

 regions: a glandular proventriculus and a muscular gizzard. 

 There are usually two caeca at the junction of the ileum and 

 colon. The lungs are attached to the walls of the coelom and 

 are provided with long sacs that extend among the viscera 

 and into some of the bones. Sense organs are well developed. 

 A middle ear is present. The carpals of the forelimb are 

 reduced by fusion and the rudimentary digits correspond to 

 digits II, III, and IV of the pentadactyl hand. In the hind- 

 limb as a result of fusions, tibiotarsal and tarsometatarsal bones 

 are formed. The elements of the pelvis are fused to form a 

 rigid structure. The sternum of flying birds is provided with a 

 deep keel for the attachment of the wing (pectoral) muscles. 



