294 CLASSIFICATION OF VERTEBRATES. 



In the majority of reptiles the teeth are either anchylosed to 

 the edge of the jaws (acrodont), or by their sides to the wall 

 of a groove (pleurodont), while in crocodiles and many dinosaurs 

 they are implanted in sockets or alveoli (thecadont) ; usually 

 the teeth are in a single row. In the snakes the teeth are 

 grooved, and in the poisonous species the grooves in one pair 

 may be very deep or completely converted into a canal, which 

 is to convey the poison into the wound made by these fangs. 

 As a rule the teeth are used for the prehension of the prey, 

 and only in the herbivorous orthopoda are they of value in 

 the comminution of food. In the turtles, and apparently in the 

 extinct edentuldus forms, the jaws are covered with an epider- 

 mal horny beak. 



Salivary glands are lacking in the marine chelonians and in 

 the alligator, while in the crocodiles they occur only on the 

 tongue. In other reptiles lingual, sub-lingual, palatine, and 

 labial glands may occur, the poison glands of ophidians being 

 modified labials. The tongue is either spatulate and immobile, 

 as in crocodiles, turtles, and a few lizards, or bifid and exten- 

 sile in other forms ; its variations of shape being of value in 

 the classification of the lacertilia. 



In the alimentary canal the most noticeable features are 

 the wide oesophagus, correlated with the swallowing of the food 

 entire, and the large intestine, frequently provided with a caecum 

 near the ileo-colic valve. In the turtles the oesophagus is 

 armed with numerous papillae pointing backward. The liver 

 is usually bilobed, but in the snakes and snake-like amphisbae- 

 nians it is unilobular and elongate. 



At no time is there a branchial respiration, the lungs being 

 the sole organs of exchange of gases. 1 The glottis is supported 

 by well-developed cricoids and arytenoids ; the trachea is long, 

 and in crocodiles and turtles may be bent into a loop. The 

 tracheal and bronchial rings are better developed than in the 

 amphibia. The lungs show variations in shape and size ; and 

 in the elongate reptiles the left lung is the smaller, and may 

 even be reduced to a rudiment (snakes). In these forms the 



1 Experiments go to show that the pharyngeal epithelium of certain North American 

 and Australian turtles has a respiratory function. 



