164 



KEPTILIA 



CLASS II 



Fig. 259. 



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 

 Python bivittatus, Caudai of the parietal and frontal. The exoccipital and opis- 



vertebra, posterior aspect. ,^ ,' r ^ i .-i ^^^ • r. i. j j 



c, ceiitrum ; d, d', Trans- thotic are lused, and the maxilia IS oiten mucn reduced. 

 TnTbrSI'.a^'yylSS^^ ^^1 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 (üropel- 



tidae), and often also on the rudi- p^p ^ ^f P Ocs 



mentary premaxillae. In the "'' 



venoüious snakes some of the 

 maxillary teeth are hollow and 

 traversed by a canal or groove 

 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 sciites 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, Etliiiioid ; /, 

 Frontal ; mx, Maxiila ; n, Nasal ; ocb, Basioccipital ; (x-l, 

 Exoccipital ; ocs, Supraoccipital ; p, Parietal ; pf. Post- 

 frontal ; pl, Palatine ; pmx, Premaxilla ; pr, Proötic ; prf, 

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

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



