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, hy, x.) 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., s ) 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 vertebras ; the neural 

 arch of the third cervical develops both pre- and post-zygapophyses (PL XXIII, fig. 6, s, z) 

 and a neural spine (ns), subcompressed like that of the axis, but narrower antero- 

 posteriorly : in both vertebras the neural spine inclines rather backward. 



The above descriptions and figures are from an immature specimen of Ichthyosaurus 

 longifrons. 



In the vertebras along about a third or more of the trunk, the neurapophysial surface is 

 continued onto the diapophysial process (PI. XXII, figs. 1 — 3, np, 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, p, 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 vertebras 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 Teh. 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, p ), bounding 



laterally the myelonal surface ( m ). This surface sinks a little deeper into the centrum as 



the vertebras recede in position, and in the caudal region it contracts both vertically and 



laterally, until it loses definition in the extreme vertebras. 



