48 BRITISH FOSSIL REPTILES. 



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 (ib., fig. 6, z, z) 

 and a neural spine (««), sul)compressed 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 Iclithi/osaurus 

 lonr/ifrons. 



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

 continued on to the diapophysial process (Tab. XVIII, 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, rf,^.), 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, ^, p.), and finally disappears at about the eightieth vertebra (ib., 

 fig. 13). 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 ((/) by contracting in 

 breadth, and take the form of narrow longitudinal grooves (ib., fig. 5, np), 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 

 laterally, until it loses definition in the extreme vertebrae. 



The articular bases of the neurapophyses undergo corresponding modifications ; the 

 joint-surface is sul)triangidar, somewhat protuberant in the anterior vertebrae ; but, after 

 the diapophysis or rib-surface gets independent, that for the neurapophysis becomes 

 longitudinal, narrow, and grooved. The neurapophysis rises, with a slight receding from 

 the vertical position, for a height usually equalling its fore-and-aft breadth ; it develops a 

 short prezygapophysis and inclines backward, with a postzygapophysial surface at its 

 under and hinder part. The pair of neurapophyses having then coalesced send upward 

 and slightly backward a subquadrate compressed neural spine, usually twice the height of 

 the the subzygapophysial part or pedicle (Tab. XVII, fig. 0, ») of the neurapophysis, and 

 with gradually augmenting height and antero-posterior breadth as far as the midpart of 

 the trunk. Towards the hind part the spines begin to lose height, but not breadth, until 



