164 



REPTILIA 



CLASS II 



Fig. 259. 



bivittaius, 



posterior 



Python 

 vertebra, 

 c, Centrum : d, d'. 



The serpent cranium (Fig. 260) differs from that of lizards in the constant 



absence of both temporal arcades ; complete ossification of the anterior part 



of the brain case ; reduction of the premaxillae ; absence 

 of a columella ; strong development of the ectopterygoid, 

 which joins the maxillary and palato-pterygoid arches ; 

 and movable connection between not only all of the last- 

 named bones, but between the squamosal and occipital 

 region, and between the rami of the lower jaw. The 

 brain cavity is very long, and its lateral walls as far as 

 the middle portion are formed by descending processes 

 Caudal of the parietal and frontal. The exoccipital and opis- 

 Trans- thotic are fused, and the maxilla is often much reduced. 



rng^raS^Xzygantru^" All the cranial bones are dense and ivory-like, and united 



by smooth sutures. 

 Acicular, recurved, acrodont teeth are usually present on the maxillae, 



premaxillae pterygoids, palatines, and dentary bones. Occasionally they are 



wanting on the palatines (Uropel- 



tidae), and often also on the rudi- ™ - f r P ^ cs 



mentary premaxillae. In the N 



venomous snakes some of the 



maxillary teeth are hollow and 



traversed by a canal or groove Pmx 



ending in a slit-like opening at the 



end of the tooth. The canal is 



connected with the duct of a 



poison gland, and through it the 



secretion of the latter is forced 



when the animal strikes. Some 



of the non poisonous snakes also 



have grooved teeth immovably 



fixed to the maxillae. 



Locomotion among snakes is 



accomplished by the lateral move- 

 ments of the vertebral column, 



assisted by the ribs, the latter 



being alternately pushed forward 



and the dermal scutes drawn after 



them. 



The Ophidia are distributed principally in the warmer regions, only the 



smaller forms extending into the northern temperate zone. By far the 



greater number are terrestrial, although some are amphibious, and a few are 



exclusively marine. About 400 recent genera and nearly 1800 species are 



known, as compared with only about 35 fossil forms, the majority of which 



are very imperfect. Most of the detached vertebrae of Cretaceous age 



originally regarded as Ophidian are doubtless Dolichosaurian, and the known 



Tertiary species are scarcely to be distinguished from those now living. 



Tolerably complete skeletons occur rarely in the freshwater Miocene of 



Oeningen and Euboea, as well as in the lignites near Bonn, and belong chiefly 



to the non-poisonous varieties. 



Typically Ophidian vertebrae (Palaeophis) from the Lower Eocene of 



Fig. 260. 



Crotalus horridus, Linn. Recent; South America, (art, 

 Articular; bs } Basisphenoid ; d, Dentary; et, Ethmoid; /, 

 Frontal ; mx, Maxilla ; n, Nasal ; ocb, Basioccipital ; ocl, 

 Exoccipital ; ocs, Supraoccipital ; p, Parietal ; %rf, Post- 

 frontal ; pi, Palatine ; pmx, Premaxilla ; pr, Prootic ; prf, 

 Prefrontal ; pt, Pterygoid ; qu, Quadrate ; sq, Squamosal ; 

 tr, Transverse or ectopterygoid), (after Claus). 



