THE IGUANODON AND H YL^OSAURUS. 
295 
the angles of the upper or basal surface of the eentriini, which support the impacted 
roots of the ribs, are therefore removed, and replaced by planes converging' towards 
each other above, and forming with the primary surfaces a longitudinal ridge on each 
side, which descends gradually to its centre in the terminal vertebrae, at the same 
time becoming more prominent as the body assumes a more hexagonal figure. In 
the vertebrae immediately adjoining the sacrum, the anterior articular surface is 
flat or slightly concave in its lower moiety, but convex above, whilst behind the 
reverse is the case, and thus the vertebral surfaces are coadapted ; in the middle 
caudal elements, the body has plano-concave facets ; the anterior then becoming de- 
pressed in the terminal vertebrae, which are thus biconcave. The expanded bases of 
the neural laminae leave a portion of the centrum uncovered inesially, above they 
contract and leave considerable tracts of the body exposed ; the posterior notch is 
twice the depth of the anterior. The elongated space ascends obliquely backwards, 
increasing in width, but is abruptly truncated ; the hinder border is in its upper half, 
while from the lower moiety of the anterior margin a thin plate extends forwards, its 
base reaching to the deep notch which separates the pro-arthrapophyses ; these re- 
ceive between them the closely approximated corresponding posterior processes which 
look outwards, and are developed on the hinder part of the base of the spine, their 
thin posterior edge being separated by a shallow notch. The free aspect of the neu- 
ropome is flat in the axis of the vertebra, but concave in the opposite diameter, the 
concavity passing upwards into the lateral surface of the spine. 
Tbe spinal canal is circular, widening slightly at each extremity. The chevron bone 
is not developed at the two first caudal intervertebral spaces in the fossil, Plate XXX. 
fig. 17 , a?, which represents four vertebrae belonging to the same individual as the frag- 
ments of the sacrum, figs. 15 and 16 : there is a marked increase in the size of the body 
to the third, and then it diminishes; that of probably the second caudal is but little 
contracted inferiorly, whilst in the third it is carinate, and encroached on posteriorlv 
by the semicircular surface descending obliquely forwards, and giving attachment to 
the anterior facet of the expanded base of the chevron bone, which is wedged into the 
intervertebral space, truncating the opposed angles of the contiguous vertebrae. The 
laminae of the chevron bone (angiopoma) coalesce at their distal extremity, and de- 
velope a long inferior spine {angiacanfha) ; they also meet above the haemal canal to 
form the expanded wedge-like base, the anterior facet of which is the largest. The 
angiopomal impressions are never in pairs, but always united into a single subtrian- 
gular rough irregular surface, the posterior of which is most extensive; the narrow 
tract separating them is deeply concave lengthwise, carinated in the more anterior 
caudal vertebree, but deeply sulcated in the more posterior elements. The chevron 
bones are continued further back than the ribs, and the angiopomal impressions are 
present on many of the hexagonal terminal vertebrae ; the posterior are the largest 
and partly subdivided by a slight median ridge. The dimensions of the first caudal of 
the above series are subjoined. 
2 Q 2 
