30 
THE EACES OF ALDABEA. 
The prsezygapophyses face towards the dorsal, the postzygapophyses towards the visceral 
side ; both are oblique, the former separated from each other by a very narrow groove, 
the latter by a crest. The continuity of this vertebra to the lumbar is indicated by the 
development of a long diapophysis flattened and dilated at the base. The second 
caudal resembles the first, approaching in form the third, the principal difl'erence from 
the first being the reduction in length of the diapophyses. Whilst still retaining the 
massive neural arch, the third, fourth, dccA fifth caudals (Plate XV. fig. B, the fourth) 
have also their centrum thicker than the preceding vertebrae, the postei'ior condyle 
being subglobular, with an obtuse hsemal ridge arising from it. Their prsezygapophyses 
are subvertical, separated by a groove, which is now wide enough for the passage of the 
blood-vessels, which branch off" from those of the spinal canal, and carry the blood 
through a foramen situated in the median line bet\A een every pair of vertebrae to or 
from the back of the tail. The postzygapophyses are likewise separated by a groove 
forming one half of the canal for blood-vessels. Diapophyses obliquely directed forwards. 
In the sixth (Plate XV. fig. C) the centrum is shortened, thicker, more distinctly com- 
pressed ; diapophyses long, directed forward ; prsezygapophyses obliquely facing dorsad, 
widely separated by a semicircular notch, and much more expanded than the post- 
zygapophyses. The spinal canal has become narrower, and is deeper than wide. The 
following vertebrae, from the seventh (Plate XIII. figs. D, D', D", ninth vert.) to the 
eighteenth (Plate XIV. figs. C, C, C", C"), are built upon the same plan, the gradual 
modifications being the following: — the centrum becomes shorter and, like the whole 
vertebra, more depressed; a haemal ridge soon becomes quite obsolete; the anterior 
glenoid cavity lower, shallower, and relatively broader ; the posterior condyle less pro- 
minent, loses ics globular form, and becomes lower and transverse ; the diapophyses 
stand at a right angle to the axis of the centrum, lose in length, retaining their relative 
breadth at the base ; the neural arch loses in substance and is depressed, nearly flat 
above ; the neural canal, again, becomes wider than deep. The last seven vertebrae are 
quite rudimentary and coalesced into a single bone. 
Possibly the extraordinary development of the neural arch of the anterior and of the 
neural arch and centra of the succeeding vertebrae is peciiliar to males of such an 
advanced age as that was from which the foregoing description is taken. In individuals 
of that sex the tail plays a very important part as an external prehensile, or rather 
steadying organ, which also diff'ers externally from that of the female in its greater 
length and by being provided with a large terminal claw. Nearly always the animal 
carries it bent sidewards under the carapace, generally towards the left side ; and there- 
fore I anticipated to find a want of symmetry in some portion of the root of the tail ; 
however, nothing of the kind can be observed. The articulations between the first four 
vertebrae are so complex that their mobility must be very limited indeed, whilst in the 
six following the anterior glenoid cavity is disproportionately large for the small size of 
the condyle ; and this seems to be the part where the lateral flexion of the tail is effected. 
