24 
THE EACES OF ALDABRA. 
Museum cannot be referred to either of these three varieties, nothing being known 
of the carapaces or any other part of the animals to which they belonged. 
0. Skull of a half-grown example in the British Museum, figured in Gray's ' Catal. of 
Shield-Reptiles' (4to), pi. 35. fig. 1. 
Variety a. 
In the adult male specimen the carapace (Plate III.) is very high, very convex in 
every direction, and nearly equally broad in front and behind. The anterior vertebral 
scute slopes rapidly from a protuberance near its hind margin down to the nuchal plate. 
The anterior and posterior free margins of the carapace are somewhat turned outwards, 
but not reverted, with shallow notches between the marginal scutes. The caudal plate 
is neither bent inwards nor outwards, its surface being a plane. The middle of each of 
the four posterior vertebrals is raised into a high hump, which gradually slopes down 
towards the margins. Also the costals participate in some measure in this peculiarity; 
but their most prominent part has an eccentric position, and is limited to the upper 
half of each scute. In all the scutes the smooth areolar portion can be clearly dis- 
tinguished from the very broad striated portion, although the striae are rather broad 
and somewhat obsolete in the vertebrals, but well-developed in the marginals. Nuchal 
plate of moderate size, rather longer than broad. The sternum (Plate IV. fig. A) is 
deeply concave, broad, with the anterior portion somewhat contracted, truncated in 
front and behind. The shell is very thick and heavy*, the substance of the caudal plates 
and of the lateral portion of the abdominals being particularly thickened and callous. 
In the large specimen, c, a section of this portion shows a thickness of three inches — 
the osseous substance, however, not being solid, but cavernous. 
The upper surface of the head (Plates I. &c II.) is covered with large irregular shields, 
of which two, covering the frontal region, are by far the largest, about thrice as long 
as broad. The neck is shorter than in any of the Galapagos Tortoises. The scutes 
on the anterior side of the fore leg are separated from the smaller ones covering the 
hinder side by a longitudinal series of large scutes, which run along the outer edge of the 
forearm. The tail is about 10 inches long, and provided with a large, flat, terminal claw. 
* Falconer, in his notes on ColossoclitJys atlas (Palaeontolog. Mem. vol. i. p. 378), states that " the thickness 
of bone in the convexity is almost in an inverse ratio to the size. The physiological reason of this is, that the 
smaller the animal, the more liable it is to injury, and it requires a greater arch to sustain it." This view is 
not contirmed by an examination of the living Tortoises ; the Aldabra species is as large as those from the 
Galapagos, and even larger than one of these latter, yet it has a much thicker shell. We shall see that the 
extinct Mascarene species agree with the Galapagos Tortoises in this respect. Perhaps the cause of this is to 
be sought in the small quantity of earthy matter contained in the food on which those animals chiefly subsist. 
A living Galapagos Tortoise in my possession preferred the petals of a Westeria to every other plant. Of course, 
by the thinness of the shell its weight is much reduced ; and these Tortoises are therefore able to walk faster and 
to carry the shell higher above the ground than the thick-shelled species. The thinness of the shell and the 
slender osseous framework of the limbs are, in fact, characters correlated to each other. 
