56 ANATOMY OF VERTEBRATES. 



The centrum of the first vertebra coalesces with that of the 

 second, and its place is taken by an autogenous hypapophysis : 

 this, in the python, is articulated by suture to the neurapophyses ; 

 it also presents a concave articular surface anteriorly for the lower 

 part of the basioccipital tubercle, and a similar surface behind for 

 the detached central part of the body of the atlas, or ' odontoid 

 process of the axis.' The base of each neurapophysis has an 

 antero-internal articular surface for the exoccipital tubercle, the 

 middle one for the hypapophysis, and a postero-internal surface 

 for the upper and lateral parts of the odontoid ; they thus rest on 

 both the separated parts of their proper centrum. The neura- 

 pophyses expand and arch over the neural canal, but meet 

 without coalescing. There is no neural spine. Each neura- 

 pophysis developes from its upper and hinder border a short 

 zygapophysis, and from its side a still shorter diapophysis. In 

 the second vertebra, the odontoid presents a convex tubercle 

 anteriorly, which fills up the articular cavity in the atlas for the 

 occipital tubercle ; below this is the surface for the hypapophysial 

 part of the atlas, and above and behind it are the two surfaces 

 for the atlantal neurapophyses. The whole posterior surface of 

 the odontoid is anchylosed to the proper centrum of the axis, and 

 in part to its hypapophysis. The neural arch of the axis de- 

 velopes a short ribless diapophysis from each side of its base ; a 

 thick sub-bifid zygapophysis from each side of the posterior 

 margin ; and a moderately long bent-back spine from its upper 

 part. The centrum terminates in a ball behind, and below this 

 sends downward and backward a long hypapophysis. 



At the opposite extreme of the elongated body, two or three 

 much simplified vertebrae are usually found blended together ; they 

 support the horny rings forming the warning rattle of the Cro- 

 talus. There is no sternum in true Ophidia. 



The skeleton of the Python (P. tigris) 1 has 291 vertebras, of 

 which the 3rd to the 251st support movable ribs. The 74 

 anterior vertebrae develope hypapophyses. The skeleton of the 

 Boa constrictor* has 305 vertebras, a hypapophysis being developed 

 from the 60 anterior ones. In the skeleton of a Rattle-snake 

 (Crotalus horridus^ with 194 vertebras, 168 support movable 

 ribs, and all these develope hypapophyses, fig. 47, h, as long as the 

 neural spines, ns. In the Naja, fig. 46, as many vertebrae have 

 the lower process, but of less length. In the Rough Tree-snake 

 (Deirodon sealer)* with 256 vertebras, a hyj apophysis projects 



1 XLIV. vol. i. No. 602, p. 123. 2 Ib., No. G30, p. 132. 

 3 Ib., No. 640, p. 135. 4 Ib., No. 638, p. 134. 



