OPHIDIA. 135 



of the fossil ophidian vertebrae, and vindicate the grounds on which some of these are 

 referred to distinct species, and others to genera of which we have no evidence of the 

 actual existence in living Nature. 



I have selected as the type of an ophidian vertebra that of a large, terrestrial, con- 

 stricting Serpent {Pi/tJion Seba), an African species, which makes the nearest approach 

 in size to some of the fossil ophidian vertebrte from British tertiary strata. The 

 vertebra figured in PI. 2, figs. 1-4, is from about the middle of the back of a 

 specimen which was twenty feet in length. In the Pythons, as in other known 

 Ophidia, all the autogenous elements, except the pleurapophyses {pi, figs. 2', 3'), 

 coalesce with one another in the vertebrae of the trunk ; and the pleurapophyses 

 (PI. 3, fig. 42, pJ) also become anchylosed to the diapophyses (ib. d) in those of the 

 tail. There is no trace of suture between the neural arch (PI. 2, figs. l-4w) and 

 centrum (c). The outer substance of the vertebra is compact, with a smooth or 

 polished surface. The vertebrae are procoelian, the cup (fig. 2 c) being deep, with its 

 rim sharply defined and most produced at the sides ; the cavity looking not directly 

 forwards, but a little downwards, from the greater prominence of the upper over 

 the lower border : the well-turned prominent ball (fig. 3 c) terminates the back part of 

 the centrum rather more obliquely, its aspect being backwards and upwards. The 

 hypapophysis (//) is developed in different degrees from different vertebrae, but 

 throughout the greater part of the trunk presents the form and proportions shown 

 in figs. 1, 4//. A vascular canal perforates the under surface of the centrum (fig. 4), 

 and there are sometimes two or even three smaller foramina. A large, vertically 

 oblong, but short diapophysis {cl) extends from the fore part of the side of the centrum 

 obliquely upwards and backwards. It is covered by the articular surface for the rib, 

 which is convex lengthwise, and convex vertically at its upper half, but slightly 

 concave at its lower half. The base of the neural arch swells outward from its 

 confluence with the centrum, and developes from each angle a transversely elongated 

 zygapophysis ; that from the anterior angle {z) looking upwards, that {z) from the 

 posterior angle downwards, both surfaces being flat, and almost horizontal. A thick 

 rounded ridge connects the anterior with the posterior zygapophysis on each side, 

 extending along the base of the neural arch. The neural canal (fig. 2, n) is narrow, 

 with a subtriedral area, and with a narrow longitudinal ridge on each side. The 

 neural spine {ns) is of moderate height, which scarcely equals its antero-posterior 

 extent ; it is compressed and truncate. A wedge-shaped process — the ' zygosphene'* 

 {zs, fig. 2) — is developed from the fore part of the base of the spine ; the lower apex 

 of the wedge being, as it were, cut off, and its sloping sides presenting two smooth^ 

 flat, articular surfaces. This wedge is received into a cavity^ — the 'zygantrum'f 

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

 smooth articular surfaces to which the zygosphenal surfaces are adapted. 



* ZCyoV, a yoke, u^/;*', a wedge. f TSiyov, and avrpot-, a cavity. 



