50 INTRODUCTION 



Maxillary bone solid Vipera. 



Maxillary bone hollowed out Ancistrodon. 



The vertebrae number 130 to 500 — in the European 

 forms 147 (Vipera ursinii) to 330 (Coluber leopardinus). 



The vertebral column consists of an atlas (com- 

 posed of two vertebrae) without ribs ; numerous pre- 

 caudal vertebrae, all of which, except the first or first 

 three, bear long, movable, curved ribs with a small 

 posterior tubercle at the base, the last of these ribs 

 sometimes forked; two to ten so-called "lumbar 

 vertebrae " without ribs, but with bifurcate transverse 

 processes (lymphapophyses) enclosing the lymphatic 

 vessels ; and a number of ribless caudal vertebrae with 

 simple transverse processes. When bifid, the ribs or 

 transverse processes have the branches regularly 

 superposed. 



The centra have the usual cup-and-ball articulation, 

 with the nearly hemispherical or transversely elliptic 

 condyle at the back (procoelous vertebrae), whilst the 

 neural arch is provided with additional articular sur- 

 faces in the form of pre- and post-zygapophyses, broad, 

 flattened, and overlapping, and of a pair of anterior 

 wedge-shaped processes called zygosphene, fitting 

 into a pair of corresponding concavities, zygantrum, 

 just below the base of the neural spine. Thus the 

 vertebrae of snakes articulate with each other by eight 

 joints in addition to the cup-and-ball on the centrum, 

 and interlock by parts reciprocally receiving and 

 entering one another, like the joints called "tenon- 



