36 
THE EACES OE ALDABEA. 
which are dilated, converge towards, and finally are merged into, the short tetrahedral 
occipital crest. 3. The tympanic case (with the mastoid *) is not produced backwards ; 
so that the posterior margin of the paroccipital is but little excised. 4. The groove on 
the lower surface of the occipital, in front of the condyle, is very shallow. A styloid 
process (Plate IX. fig. B, a) projects obliquely outwards on each side of the occipital 
condyle, and appears to be quite peculiar to this species. 5 to 11. In these points the 
skull of the present species agrees completely with that of T. elephantina. 12. The 
palate is deeper than in any of the other species, with a very low median crest along its 
bottom. The distance between the foramina palatina is conspicuously less than that 
between a foramen and the anterior extremity of the vomer. 13. Anterior surface of 
the tympanic pedicle very slightly concave. 14. Lower jaw as in T. elephantina and 
T. daudinii. 
Cervical vertehrce. — On comparing the neck-vertebrae of T. ponderosa with those of 
T. elephantina we find the greatest similarity between them. All the distinctive 
characters by which the latter differs from the Galapagos species are repeated here ; 
and the first and seventh are the only vertebrae which exhibit peculiarities indicative of 
specific distinctness. In the atlas (Plate XIII. fig. C) the lateral portion of the neural 
arch is very broad, much broader than the zygapophysis, which is nearly as long as that 
part of the bone which forms the roof of the arch. The centrum (odontoid process) is 
longer than broad, a tectiform body with the visceral surface compressed and with a 
broad articular surface in front and behind. The fourth is the first vertebra of the 
series which shows a deep hollow in the neural arch behind the anterior zygapophysis 
to receive the end of the zygapophysis of the preceding vertebra. The dorsal surface of 
the sixth has a very low median ridge, but no crest ; and in the seventh the " summit " 
is raised quite to the level of the top of the anterior zygapophyses. 
Measurements of cervical vertebrae : — 
2nd. 
3rd. 
4th. 
5th. 
6th. 
7th. 
millim. millim. millim. miHim. millim. millim 
Length of centrum 
36 
53 
51 
63 
66 
57 
Depth of centrum in the middle 
27 
26 
24. 
26 
33 
43 
Horizontal width of middle of centrum 
12 
13 
13 
15 
22 
24 
"Width of anterior condyle 
12 
16 
Width of anterior glenoid cavity 
22 
22 
28 
30 
Width of posterior condyle 
19 
20 
24 
28 
Width of posterior glenoid cavity , 
15 
35 
Distance of outer margins of anterior zygapophyses 
24 
31 
31 
33 
34 
32 
Distance of outer margins of posterior zygapophyses . 
. 27 
29 
30 
30 
30 
42 
Dorsal vertehrce. — Also this part of the vertebral column differs but slightly from 
that of T. elephantina., except in the singular shortening of the eighth vertebra, which 
is coalesced with the two following, the sutures between them having nearly entirely 
* This bone has been lost by the preparator. 
