750 
PROFESSOR OWEN ON THE MEGATHERIUM. 
the level of the centrum (c) ; the anterior zyg-apophysis (z) seems to be supported 
by a similar process {d”) from the upper end of the diapophysis, the point of which 
projects a little beyond the end of the zygapophysis {z, fig. 37) '• I refrain from giving 
names to these upper and lower processes, because they really are nothing more 
than mere subdivisions of the diapophyses, and characterize the vertebrae of a few 
genera of a single order of Reptiles. 
In the Boas and Pythons (fig. 24-27) the divisions (d" and d') of the diapophysis 
are wholly wanting ; that process is entirely occupied by the articular surface for the 
rib, which is vertically oblong, convex in the axis of the centrum, and convex verti- 
cally at its upper half, but concave vertically at its lower half. The base of the 
neural arch swells outward, from its confluence with the centrum, and developes 
from each angle a zygapophysis, which is much longer transversely than in any of 
the preceding genera of Serpents (fig. 27, 'z,z'). The zygosphene (fig. 25, zs) is nar- 
rower in proportion to its height, and the zygantrum (fig. 26, 2 a) is correspondingly 
deeper and more circumscribed. In an African Constrictor {Python regius), which 
measured 15 feet 6 inches in length, and had 348 vertebrae, the first seventy had long 
hypapophyses, which afterwards subsided into the obtuse ridge and tubercle shown 
at fig. 27, hy. 
Of the value of the minute and apparently insignificant modifications of the verte- 
bral processes above described, some idea may be formed by their application to 
determine the nature and affinities of the great Serpent, of which the vertebrae have 
been found fossil in the eocene tertiary deposits of Kent and Sussex. The veritable 
Ophidian nature of the fossils in question is demonstrated, not only by the super- 
added zj^gosphenal (2.?, fig. 45) and zygantral (za, fig. 46) articulations, but by the 
solidity of the zygosphene, by the size and form of the centrum, by those of its 
articular cup (c, fig. 45) and ball (c', fig. 46), and of its hypapophysis (hy ) ; and also 
by the size and prominence of the diapophysis {d). The largest vertebrae, probably 
from about the middle of the body, as compared with the vertebrae from the same 
part of the skeleton of a Python Sebce, 20 feet in length, are longer in proportion to 
their breadth, and the cup and ball of the centrum are larger; the hypapophysis (h) 
is more produced, and there is a second smaller hypapophysis close to the anterior 
part of the under surface of the centrum, which in most of the large vertebrae is con- 
nected by a ridge with the hinder and normal hypapophysis (fig. 47, hy), but in a 
few vertebrae is not so connected. The articular cup and ball are less obliquely 
placed upon the extremities of the centrum, being nearly vertical. The rim of the 
cup is sharply defined, and is more produced from between the bases of the diapo- 
physes ; a deeper and narrower chink intervening than in the Python. The trans- 
verse diameter of the cup (c, fig. 45) is greater than that of the zygosphene {ib., zs ), — 
a proportion which I have not found in the vertebrae of any existing genus of Serpent, 
in which the base of the zygosphene always equals at least the parallel diameter of 
the articular cup. The articular part of the diapophysis {d) is more produced out- 
