OF HYPSILOPHODON FOXIT. 
1047 
and becomes an inconspicuous tubercle wbicb soon disappears. Where this occurs the 
length of the centrum is slightly increased. Its shape also becomes cylindroid, with a 
contracted middle and swollen ends, flattened slightly, laterally, and inferiorly, where 
also an oblique facet at each end marks the attachment of the chevrons (Plate 74, 
fig. 13). The spinous processes and chevrons are reduced much more slowly than are 
the transverse processes. In a rudimentary form they persist to the end of the tail ; 
where most developed, in the front half of the tail, the spinous processes have the shape 
of flat oblong blades. The chevrons are longer than the spinous processes; their 
articular end is stout, and when well preserved is wedge-shaped, the anterior facet 
of the wedge being slightly the larger ; their free end is expanded and flattened; 
and intermediately their shaft is contracted and slender. 
In the part of the tail shown in Plate 74, fig. 13, the average length of the centrum 
is 25 millims., that of the spinous process is 35 millims., and that of the chevron bones 
51 millims. The longitudinal streaks in this figure are ossified tendons. 
The changes of shape of its articular ends and of the length of the vertebral centrum, 
in passing from the cranial to the caudal end of the centrum, add another to the 
already numerous refutations of the dictum which for many years was a great hindrance 
to the reconstruction of the dinosaurian skeleton, viz.: that the shape of its ends 
and the length of the centrum are constant throughout the column. The double costal 
articulation is repeated in Crocodilians, but not the opisthocoelous form of the centrum 
in the neck and front of the back. The great depth of the tail is probably in adapta¬ 
tion to swimming. 
Ribs .—Pibs (pleurapophyses) are borne by all the praesacral vertebrae. (This state¬ 
ment does not apply to the two first vertebras, respecting which information is 
still wanted.) 
In the neck the riblets are short, their vertebral end is forked, the branches lie in a 
nearly vertical plane, and they articulate with corresponding upper and lower vertebral 
transverse processes. Their ventral or free end is extended antero-posteriorly. 
In the anterior dorsal region the capitular branch is long. Near the middle of the 
back the tubercular branch is reduced to a mere tubercle placed where the long, 
slender capitular branch or neck and the shaft of the rib join. Ribs from the pos¬ 
terior dorsal region show a reduction of the length of the neck with an approxi¬ 
mation of the head to the tubercle, until in the loins both blend in a single terminal 
articular facet attached to the end of the transverse process. The form and the 
arrangement of the ribs in the neck and back are closely repeated in extant Crocodilia, 
but in the loins there is a small peculiarity to which allusion has been already made—- 
the anchylosis of the rib (pleurapophysis) with the end of the transverse process, their 
junction being marked by a nodal swelling. I have seen this in three skeletons of 
adult Hypsilojohodon, but not in those of Crocodilia. 
6 s 
MDCCCLXXXIL 
