612 WILLIAM R. SMELLIE ON 



the ribs, is concave both longitudinally and transversely except for the longitudinal 

 ridge between the nutritive foramina. This ridge is inconspicuous in anterior 

 cervical regions, but, towards the posterior, the nutritive foramina become more widely 

 separated and the consequent increase in the width of the dividing ridge causes the 

 ventral side of the centra to become actually convex from side to side. By the time 

 pectoral and dorsal vertebrae are reached this process has caused the centra to 

 assume a much more circular outline. The sides of the cervical centra above the 

 rib facets are gently concave. The centra in middle cervical regions have rugosities 

 and tubercles situated both on the sides and under surfaces, close to the posterior 

 ends. These are less well marked in the anterior ends. The rugosities persist into 

 dorsal regions and form definite plications. 



Throughout the neck the neural arches are fused to the centra, but the sutures 

 can be traced, most easily in the anterior region, as concave lines coming well down 

 the sides of the centra. The neural arch occupies the whole length of the centrum, 

 and bears strong anterior and posterior zygapophyses (a.z. and p.z.), the latter pro- 

 truding a greater distance over the end of the centrum. In the anterior part of the 

 neck the anterior and posterior zygapophyses are connected by a sharp ridge, 

 concave upwards, which persists, although becoming less sharp, in the posterior part 

 of the neck. 



The neural spines (n.s.) are fairly perfect in cervical regions. The spine on the 

 first cervical is very similar to that on the axis, and half way between the tip and 

 the line of the zygapophyses there is a pronounced rough ridge sloping downwards 

 and forwards. This ridge persists in the other anterior cervicals, but is less apparent, 

 and is represented finally by a few tubercles half way up the side of the spine. Both 

 anterior and posterior borders of the spine are very sharp. 



The cervical ribs (c.r.) are noteworthy for their variation. They are greatly 

 compressed from above downwards, especially in middle cervical regions. Some 

 might be described as straight, flat rods of bone with slight rounding at the anterior 

 end of the extremity. Others show the anterior angle (a. a.), relic of the double 

 head, almost as prominently as the cervical ribs of Picrocleidus . These last occur 

 chiefly in the anterior part of the neck, where the ribs are short. Towards the 

 posterior the ribs become long and straight and lack the terminal expansion present 

 in the others (text-fig. 6, A). Each rib broadens at its junction with the centrum, 

 and occupies practically the whole length of the centrum in anterior cervical regions ; 

 but the posterior cervicals have a distinct space between the anterior edge of the 

 rib facet and the end of each centrum. Going from the posterior end of the neck 

 forwards, the cervical ribs show a progressive degree of fusion. Thus, on the left 

 side, from the first to the nineteenth cervical, the ribs are closely fused to the centra 

 (text-fig. 2) and the suture closed and hard to trace. The next three ribs are fused 

 imperfectly, leaving the suture gaping. The next has been slightly fused and has 

 separated along the suture, while the remainder are quite separate and show no 



