324 



CLASSIFICATION OF VERTEBRATES. 



of the lower jaw are united by ligament at the symphysis, and are capable 

 of wide separation, which, together with the loosely articulated cranial bones, 

 allows of great increase in the size of the oral opening. 



In only the peropoda are rudiments of hind limbs visible as small 

 stumps having small claws on either side of the vent. The pelvic girdle 

 occurs as a rudimentar\- structure, unconnected with the vertebral column, 



in the peropoda and Typhlops. In 

 the latter but the rudiments of the 

 girdle occur, in the former the pelvis 

 is represented by a slender bone with 

 which two diverging bones are articu- 

 lated below. 



The tongue of the snakes is 

 deeplv forked, and is retractile into 

 a sheath on the floor of the mouth. 

 It can be protruded when the mouth 

 is closed, thanks to a groove in the 

 edge of the lips. The poison glands 

 which occur in certain snakes are 

 modified labial glands. Sometimes 

 thev are greatly enlarged, and may 

 extend backwards into the throat. 

 They are so placed that the action of 

 the muscles which close the jaw will 

 force the poison out through a duct into a groove in the modified teeth 

 which serve as poison fangs. In the proteroglyphs these grooves are open; 

 but in the solenoglj-phs the edges of the groove meet, so that a poison 

 canal is formed inside the tooth. The stomach is long, and the intestine 

 has few convolutions. The trachea is very long, and often has respiratory 

 chambers in its course. The left lung is rudimentar\", the right verj- long 

 with an air reservoir at the end. 



Locomotion is efEected by the lateral bending of the vertebral column 

 and by the ribs, which can be moved forward and back ; and as these are 

 attached to the abdominal scutes these latter can be moved, and as thev 

 catch everj' irregularity of the surface, the animal is able to propel itself. 

 Snakes are all carnivorous, the majority feeding upon vertebrates, some 

 killing their prey by poison, some by crushing it. It is swallowed whole. 

 Most snakes lay eggs, which are large and enclosed in a leather)- shell; but 

 a large number are viviparous. Snakes, of which nearlv 2,000 species have 

 been described, have their metropolis in the tropics. In colder climates 

 they undergo a hibernation during the winter. The earliest snakes appear 

 in the cretaceous : but few fossils of the group are known, and these chiefly 

 l)y vertebrje, skulls being very rare. 



Sectio.v I. COLUBRIFORMIA. Snakes with the supratemporal 

 overlapping the cranium ; maxillary bone horizontal, not erectile ; teeth not 



Fig. 319. Rudimentary pelvis and 

 hind limb (^A) of Sienostoma macro- 

 lepis and (^) of Boa, after FUr- 

 bringer. y, femur ; ?7, ilium ; ipj 

 ' iliopectineum ; ' /, pubis ; /, tibia. 



