OPHIDIA. 53 



(fig. 3, zd) — excavated in the posterior expansion of the neural arch, and having two 

 smooth articular surfaces to which the zygosphenal surfaces are adapted. 



Thus the vertebrae of Serpents articulate with each other by eight joints in addition 

 to those of the cup and ball on the centrum ; and interlock by parts reciprocally 

 receiving and entering one another, like the joints called tenon-and-mortise in carpentry. 

 This is the most conspicuous, but is not the peculiar characteristic of an ophidian 

 vertebra ; the zygosphene {zs) and zygantrum {zd) being developed in certain Lacertians, 

 e. g. the genus Iguana (T. XIII, figs. 34, 35), but here the articular diapophysis (fig. 33, d) 

 is much smaller, and forms a simple, convex, sessile tubercle ; the hypapophysis is 

 wanting : the zygosphene (fig. 34, zs) is deeply notched anteriorly, and the zygantra 

 (fig. 35, za, zd) are shallow, and separated from each other behind : a thick rounded 

 eminence extends backwards from the diapophysis to the ball on the back part of the 

 centrum (fig. 36) ; and that ball is a transverse ellipse (fig. 35), not hemispheroid, as 

 in the Ophidia. 



With regard to the specific distinctions which may be deduced from the characters 

 of the vertebrae of Serpents, it is requisite first to determine the extent to which those 

 characters vary in the vertebral column of the same species. 



The atlas and axis are modified in the same degree as in the Crocodilia, with the 

 addition of the entire suppression of their pleurapophyses. The atlas (T. XIV, figs. 38, 39) 

 has two hypapophyses, one behind the other, as we shall find to be the case in other 

 vertebrae of one of the great fossil Serpents. The normal hypapophysis {h, fig. 39), 

 answering to that marked ca, ex in the woodcut, fig. 8, p. 85, is autogenous and 

 wedge-shaped, as usual in the Rcptilia ; and is articulated on each side to a small 

 portion of the neurapophysis {n) ; it also presents a concave articular surface anteriorly 

 (fig. 38,/<) for the lower part of the basioccipital tubercle, and a similar surface 

 behind for the detached central part of the body of the atlas (fig. 40, ca), which is here 

 confluent with that of the axis (fig. 40, ex), forming the so-called odontoid process of 

 that vertebra ; its Ophidian peculiarity being the development of an exogenous 

 hypapophysis (//) from its under and back part, like the posterior hypapophysis of the 

 succeeding vertebrae. 



The base of each neurapophysis of the atlas (fig. 38, n) has an antero-internal 

 articular surface for the exoccipital tubercle, and a postero-internal surface for the 

 upper and lateral parts of the odontoid {ca), besides the small median inferior facet for 

 the detached hypapophysis {h) : they thus rest on both the separated parts of their 

 proper centrum. The neurapophyses expand and arch over the neural canal, but 

 meet without coalescing. There is no neural spine. Each neurapophysis developes 

 from its upper and hinder border a short zygapophysis {z) : and from its side a still 

 shorter diapophysis {d). 



The axis or second vertebra of the trunk with the partially coalesced body of th.p 

 atlas or ' odontoid,' is represented at fig. 40, T. XIV. 



The odontoid presents a convex tubercle anteriorly, which fills up the articular 



