134 DR. J. E. GRAY ON THE CHELYDID. [Mar. 8, 
** Head elongate, narrow. Alveolar surface of the upper jaw with 
two sharp edges; ridges parallel to the edge of the jaw and 
the opening of the inner nostrils; the hinder ridge thinnest, 
and nearly on the margin of the opening of the inner nostrils. 
Chelonemys. 
2. PopocNEMIS DUMERILIANA, Wagler, N. Syst. Amph. 135; 
Gray, Cat. 62, t. 28. 
Emys expansa, Cuvier, Oss. Foss. v. pt. 2, t. 11. f. 9-12 (skull), 
not Dum. et Bib. 
The two species of Podocnemis are well distinguished by the shape 
of the dorsal disk, and by the smaller size and more elongated shape 
of the head of the animal; but I am not aware that the distinction 
which exists between the two species in the structure of the skull 
has been before recorded. 
In the British Museum there are two skulls of each species, re- 
ceived from Mr. Bates, from Ega. And it is important to observe 
that the skull figured by Cuvier (Oss. Foss. vol. v. part 2, t. 11. 
f. 9-12) as Emys expansa is one species, and that figured by Wagler 
(N. Syst. Amph. t. 4) as Podocnemis expansa and by me in the 
‘Catalogue of Shield Reptiles in the British Museum’ (t. 37. f. 1) 
is the other. 
They are very similar externally ; but the skulls of P. expansa are 
much larger and much broader, compared with their length, than 
those of P. dumeriliana. The frontal plate of the latter species is 
much larger compared with the size of the head, and it is also longer 
and narrower, than the same plate in P. expansa. The ridges on 
the alveolar surface of the upper jaws of the two species are very 
distinct ; and this distinction seems permanent, as it has been ob- 
served on three specimens of the skull of P. expansa. 
In P. expansa there are three ridges; the two front ones are 
nearly parallel, and they diverge from the centre towards the hinder 
end of the maxillary edge, so that they are at an angle both with the 
maxillary edge and with the edge of the internal nostrils; the front 
of these two ridges only half the length of the hinder one. The third 
ridge is scarcely raised, broad, rugose on the edge of the concave 
cavity for the internal nostril. 
In P. dumeriliana there are only two ridges, both of which are 
parallel to the edge of the jaw and the edge of the palatine cavity, 
which are nearly parallel to each other. The front of the two ridges 
is much the strongest and largest; the hinder one is shorter, nar- 
rower, but well developed and very near the edge of the palate-open- 
ing, as is well represented in Cuvier’s figure of the skull of Emys 
expansa (Oss. Foss. t. 11. f. 9-12). 
In the skull of the older P. eapansa the two front ridges become 
higher, more tubercular, and do not increase in length with the size of 
the skull; so they appear shorter in proportion, and the tubercular 
ridge on the margin of the opening to the internal nostril is less 
distinct. The skull of a very young specimen of this species is 
