REPTILES 197 



loosely articulated with one another and with the cranium, so that the 

 mouth can be stretched to a remarkable degree. The vertebra? are 

 procoelous, giving the body great flexibility, those in front of the anus, 

 except the atlas, all bearing ribs, those back of the anus being without 

 ribs but bearing long transverse processes. Acrodont teeth are always 

 present; in poisonous snakes usually a single pair of maxillary teeth are 

 much elongated and form the poison fangs, of which 3 types are 

 observed in American snakes: i. Those of the opisthoglyph snakes, 

 belonging to the family Colubridcc, in which there are one or more 

 pairs of elongate and grooved but not perforated fangs at the rear of the 

 upper jaw; (2) those of the elapine snakes, belonging to the family 

 ElapidcB, in which there is a pair of short rigid fangs in the front of the 

 upper jaw, which are perforated by a poison canal and also grooved on 

 the outer surface; and (3) those of the viperine snakes, belonging to 

 the family Crotalidce, in which there is a pair of very long perforated 

 fangs in the front of the upper jaw, which lie against the roof of the 

 mouth when not in action. The tongue is long and forked and protrac- 

 tile; it can be thrust out of the closed mouth, and is an important 

 tactile organ. 



The scales are arranged in a definite number of rows, and are 

 either keeled (i.e., with a median longitudinal ridge called the keel) 

 or not keeled. The scales on the ventral surface differ in most snakes 

 from those on the dorsal and lateral surfaces, forming a single row of 

 transverse plates called the ventrals or gastrosteges where they occur in 

 front of the anus, and either a single or a double row called the sub- 

 caudals or urosteges where they lie back of it. The ventrals bear a 

 relation to the ribs and are of assistance in locomotion inasmuch as 

 the animal can move them and thus advance the body over a roughened 

 surface; the ventral immediately in front of the anus is either divided 

 in two or not, and is called the anal plate. 



Habits and Distribution. — Most snakes are oviparous, although 

 many species are ovoviviparous. They feed almost exclusively on 

 live animals or on animals they have recently killed; a few tropical 

 snakes, however, are herbivorous. The poisonous snakes kill their 

 prey with their poison and the constrictor snakes by crushing before 

 swallowing it; other snakes do not stop to kill the prey but swallow 

 it alive. Snakes live in a great variety of localities, some species 

 being terrestrial, others arboreal; a large number are aquatic, some 

 living in the open sea. ]Many species are brightly colored, especially 

 in the tropics, but in none has metachrosis, so common among lizards, 

 been observed. 



