90 
FOSSIL REPTILIA OF THE 
length of this rib is a little more than the vertical diameter of the centrum. The lateral 
surfaces of the centrum of the axis-vertebra are antero-posteriorly concave, of greater 
extent behind the rib-processes ; vertically they describe a convex curve converging 
from each side to an inferior medial ridge. This ridge is interrupted, anteriorly, as if 
that end had been obliquely cut off, forming a roughish subconcave facet for the hind half 
of the base of the second or axial hypapophysis (PI. XXIII, fig. 5, a;.) This element 
is barely half the size of the one in front, is conical, the apex downward; the base 
is divided into the surfaces respectively joined to the contiguous hypapophysial facets of 
the atlas and axis. The neurapophyses converge as they rise and coalesce to form the 
base of a neural spine (PI. XXIII, fig. 6, n s), the antero-posterior extent of which equals 
the height — a proportion which distinguishes that part of the second vertebra. Postzyga- 
pophyses (ib., -■) are developed from the base of the spine which overlap and articulate with 
the prezygapophyses of the third cervical vertebra. 
Thus, at the fore part of the vertebral column, the neural arch presents the three 
following modifications : — In the atlas the neurapophyses remain distinct and develop 
neither post-zygapophyses nor neural spine ; in the axis they coalesce, develop the post- 
zygapophyses and a lofty spine, broader than those in the succeeding vertebrae ; the neural 
arch of the third cervical develops both pre- and post-zygapophyses (PI. XXIII, fig. 6, z, z) 
and a neural spine (ws), subcompressed like that of the axis, but narrower antero- 
posteriorly ; in both vertebrae the neural spine inclines rather backward. 
The above descriptions and figures are from an immature specimen of Ichthyosaurus 
lougifrons. 
In the vertebrae along about a thii'd or more of the trunk, the neurapophysial surface is 
continued on to the diapophysial process (PI. XXII, figs. 1 — 3, n'jj, d)- This process next 
becomes distinct (ib., fig. 4, d) ; and, as the parapophysis continues to be developed, the 
presence of the pair of tubercles, d, f, near the fore margin of the side of the centrum cha- 
racterises that part as far as the fortieth or forty-fifth vertebra in Ichthyosaurus communis. 
In this course both processes gradually descend (ib., figs. 5, 7, d,p), but the diapophysis 
more rapidly, until it coalesces with the parapophysis, forming therewith an oblique 
ridge or rising. In the caudal vertebrae the ridge gradually contracts to a rounded 
tubercle (ib., figs. 9, 11, d,p), and finally disappears at about the eightieth vertebra (ib., 
fig. 14). At this part of the column, in Ich. communis, the abrupt bend or dislocation of 
the caudal series commonly occurs ; and here three or four of the centrums become more 
compressed than either those that precede or those that follow them, and their lateral 
margins are raised, as if by forcible compression. 
The neurapophysial facets become detached from the diapophyses {d) by contracting in 
breadth, and take the form of narrow longitudinal grooves (PI. XXII, fig. 4, ^), bounding 
laterally the myelonal surface (?«). This surface sinks a little deeper into the centrum as 
the vertebrae recede in position, and in the caudal region it contracts both vertically and 
lateralhq until it loses definition in the extreme vertebrae. 
