TRIONYX LABIATUS. 
Testee ossece mensura. 
unc. lin. 
Testa? dorsalis longitudo . 6 6 
- latitudo . 6 4 
Sterni longitudo. 6 4 
- latitudo. 7 4 
Of this beautiful and hitherto undescribed species of Trionyx , I obtained 
some time since a living specimen which had been brought from Sierra Leone. 
I was at first doubtful whether it might not he considered as a variety of Tr. 
cegyptiacus, as the form and the markings have considerable resemblance, 
although the colours differ materially. The situation and form of the sternal 
callosities are also, in a great measure, similar. Having had drawings made 
from the upper and under sides during life, I determined on sacrificing the 
soft parts after death, for the sake of obtaining a perfect skeleton; and I then 
found that the characters of the bony shell exhibited the most marked differ¬ 
ences from the species above alluded to, and indeed from all others. Inde¬ 
pendently, then, of the distinctions in the colour of the skin, we find that the 
first dorsal vertebra, or, as Mr. Gray has very well named it, the os nuchale , is 
separated from the bony shell, in which respect it differs from Tr. cegyptiacus 
and Tr. javanicus ; from both of which also, as well as from Tr. subplanus, it is 
readily distinguished by the extent of the free portion of the ribs, which is 
nearly equal in length to the connected part which forms the solid shell. From 
the osseous parts of Tr. carinatus, as figured by Geoffroy, its distinction is not 
at first sight so obvious; hut, upon examination, it will be found that even the 
bony shell affords sufficient characters to show that it is specifically distinct. 
The callous portion of the os nuchale is proportionally much longer and larger 
in the present species. The anterior calli of the sternum in Tr. carinatus are 
nearly half as broad again at the inner part as at the outer; in Tr. labiatus they 
are nearly of the same breadth throughout. The toothed margins of the ster¬ 
nal bones are also much more divided, as well as very differently formed in 
the latter. But in the character from which the approximating species has 
