240 



Order CHELONIA (Turtles and. Tortoises). Body within a bony case, 

 which is covered with epidermal horny plates of "tortoise-shell," 

 and which consists of two joined shields (dorsal arched carapace 

 and ventral flat plastron). Generally the carapace is formed 

 from dermal scutes combined with the thoracic vertebrae (upper 

 parts) and the flattened ribs ; the plastron consists of dermal 

 bones. Jaws without teeth, but with horny plates. 



Order CROCODILIA. Exoskeleton consists of epidermal horny scales 

 overlying dermal bony plates. Maxillae, palatines and ptery- 

 goids are united along middle line on roof of mouth ; and in- 

 ternal nares are placed far back on hinder portion of pterygoids 

 and close to the glottis. External nares (nostrils) re at tip of 

 snout. There is an os transversum between maxilla and ptery- 

 goid. Teeth in sockets (thecodont) on premaxillae, maxillae 

 and dentaries. Cervical vertebrae have two-headed ribs. The 

 vertebral portions of the thoracic ribs have small uncinate 

 processes; the sternal portions join (and form) the sternum. 

 There are " abdominal ribs " (loose paired membrane bones). 

 Heart has two auricles and two ventricles. 



Examples, Crocodilus. The first mandibular tooth bites 

 into a pit, the fourth into a groove of the 

 upper jaw. 



Alligator. The first and fourth mandibular 

 teeth bite into pits of the upper jaw. 



CLASS AVES (BIRDS). 



State the Chief Distinctive Characters of Birds. 



The epidermal exoskeleton consists of feathers and 

 scales. Skin glands are notably absent ; but there is 

 generally an oil or preen gland above the base of the tail. 

 The bones are light, being mostly marrowless and containing 

 air ; fusion is also characteristic. The skull is high and 

 arched ; almost all the bones are ankylosed, the sutures 

 between them obliterated. The long premaxillae form the 

 greater part of the beak. The orbits are large, and, as in 

 many Reptiles, there is no intervening cranial cavity ; 

 there is a thin bony interorbital septum. A membrane 

 bone, the basitemporal (representing the parasphenoid) 

 covers the basisphenoid. There is one (basioccipital) 

 condyle. The jaws, without teeth in modern birds, have 

 horny sheaths ; the lower jaw articulates with the tn.onthlc, 

 quadrate. The neck is long ; the cervical vertebrae are 

 heterocoelous (see page 197) and have reduced ribs fused on 

 their sides. The thoracic^vertebrae tend to fuse. The 



