PROFESSOR OWEN ON THE MEGATHERIUM. 
751 
wards, and less extended vertically in Paloeophis than in Python, and it is uniformly 
convex; a ridge is continued from its upper end obliquely forwards; as far as, but 
not beyond, the apex of the anterior zygapophysis (x), forming the angle between 
the lateral and anterior surfaces, whilst the horizontal articular facet forms the third 
surface of that three-sided conical process. In the Python the non-articular part of 
the same zygapophysis is convex, and the process is much more extended outwardly ; 
the proportions of the zygapophysis in the Paloeophis more resemble those in the 
Coluber and Hydrus, but dilfer from these, as also from N^aja and Crotalus, in the 
non-extension of the diapophysial point beyond the articular suiface. 
A ridge or horizontal rising of the bone extends from the anterior to the posterior 
zygapophysis, but is more or less blunted or subsides midway, and is by no means 
so produced outwards as in Python ; in this respect more resembling that in Coluber 
and Hydrus. Below the middld" of this ridge, on a level with the upper surface of 
the centrum, there is a short, nearly parallel rising in Paloeophis (fig. 44). The 
zygosphene (fig. 45, z 5) is slightly excavated anteriorly, and shows no trace of the 
tubercle which characterizes the middle of that surface in the Python (fig. 25) ; it is 
also broader in proportion to its height. But perhaps the most characteristic feature 
of the vertebrae of the Paloeophis, is the peculiar production of the posterior border 
of the neurapophysis into an angle {n, fig. 44) directed upwards, outwards, and 
backwards: there is no trace of this process in the Hydrus (fig. 33), or the other 
existing genera of Serpents. The posterior zygapophysis resembles, of course, the 
anterior one in its much less extent, especially transversely, as compared with that 
in the Python ; and the posterior border of the neurapophysis (fig. 44, z' , n) rises from 
its apex vertically, or a little inclined outwards and backwards, giving a squarish 
form to the surface of the neural arch in which the zygantra {za, fig. 46) are ex- 
cavated ; these cavities, in proportion to the articular ball beneath, are smaller and 
less deep than in the Python, or any other existing genus of Serpent. The sloping 
sides of the neural arch above the zygapophysial ridge are more concave than in 
Python, and so resemble those parts in Coluber and Hydrus. The latter genus 
(fig. 33) and Crotalus (fig. 36) most resemble Paloeophis in the proportions of the 
neural spine {ns)-, this part, however, in Paloeophis differs from that of Hydrus in 
having its base coextensive with the supporting arch, springing up from the fore- 
part of the zygosphene, whilst the zygosphene entirely projects forwards, clear of the 
base of the spine in Hydrus, as in Python, Coluber, and Naja ; but in Crotalus the 
base of the spine has the same antero-posterior extent as in Paloeophis. The neural spine 
had been more or less fractured in every detached specimen of the brittle crumbling 
vertebrae of the Paloeophis Typlueus from the Bracklesham Clay ; — only one specimen, 
which I carefully worked out in relief, from a mass of matrix, after imparting some 
of its original tenacity to the substance of the bone, afforded a true idea of the pe- 
culiar character of these Ophidian vertebrae, which is afforded by the great height of 
the neural spine, as shown in figure 44. 
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