34 Descriptions of Preparations. 



especially at the sides and above ; and the enarthrosis thus formed 

 between each pair of vertebrae being supplemented by the arthro- 

 dial joints developed above, mesially by the neurapophyses, and 

 laterally by the zygapophyses, great flexibility is conferred upon 

 the trunk as a whole, whilst great security against the dislocation of 

 individual vertebrae from their fellows is obtained at the same time. 

 In venomous snakes the hypapophyses are greatly developed and 

 present in all the trunk vertebrae ; in non- venomous snakes they 

 may be, as in the Python, aborted for the posterior two-thirds or 

 three-fourths of the length of the trunk. They are always present, 

 and mostly bifid in the caudal region. 



There are no very important difi'erences between the two cervical 

 vertebrae of Ophidians and the atlas and axis of any other Reptiles. 

 The first of the two cervical vertebrae has no neural spine, con- 

 trasting herein with that of the Crocodilidae, but coinciding with 

 that of the rest of the class. Its neurapophyses neither anchylose 

 with each other above, nor with any other autogenous vertebral 

 element ; together with the anterior hypapophysis of the vertebra 

 and its centrum which is more or less confluent with that of the 

 'axis,' the neurapophyses form the transversely elongated and 

 shallow articular cavity for the trefoil or heart-shaped occipital con- 

 dyle which is usual in Eeptiles, and still retained in the less highly 

 specialized Birds "". The centrum of the ' atlas,' the so-called ' odon- 

 toid,' comes into apposition and partial coalescence in the way of 

 amphiarthrosis with the concavity of a crescent-shaped surface on 

 the body of the second cervical vertebra. Its homology with the 

 centra of the succeeding vertebrae, which is proved by other con- 

 siderations, is illustrated by the presence on its upper surface of a 

 foramen on either side of the middle line just as in these vertebrae. 

 A second hypapophysis is articulated to its inferior surface and to 

 that of the 'axis' where they come into amphiarthrosis. The axis 

 resembles the vertebrae placed posteriorly to it in having its 

 hypapophysis anchylosed with its centrum, and in having a well- 

 developed ball difierentiated by a slight constriction both from the 

 hypapophysis and from the posterior end of the centrum for 

 articulation with the socket in the anterior end of the vertebral 

 centrum next behind it. 



" See Parker, Osteology of Gallinaceous Birds and Tinamous, Zool. Soc. Trans. 

 1862, p. 183. 



