SOFT- TORTOISES. 
99 
which are covered with a raised sculpture of variable form, and are quite devoid of 
horny shields. The lower shell, or plastron, is always very imperfectly ossified, 
and completely separate from the carapace; while the carapace never has a complete 
series of marginal bones, and passes at its borders into a soft expansion of skin, 
from which the name of the group is derived. If marginal bones occur at all, 
they are confined to the hinder border of the shell, and are unconnected with the 
ribs; having, in fact, nothing in common with the bones so named in other tortoises, 
and being doubtless of independent origin. In being unconnected with the plastron, 
the pelvis resembles that of the S-necked group, and the head is retracted by a 
similar S-like flexure of the neck in a vertical plane. In regard to the mode of 
articulation of the lower jaw with the skull, and likewise in the presence of a 
notch in the hinder border of its tympanic ring, the soft-tortoises again resemble 
the group last mentioned; although in the general form of the skull and the 
conformation of the palate they come nearer to the Side-necked group. A distinc¬ 
tive peculiarity of the skeleton is to be found in the presence of at least four joints 
in the fourth toe of each foot. Externally, the soft-tortoises are characterised by 
their long necks, which, together with the head, can be completely withdrawn into 
the shell, and also by the proboscis-like snout, and the thick fleshy lips concealing 
the jaws. The ear is completely concealed; and each foot, as indicated by the 
scientific name of the group, has but three claws, which are borne by the three 
inner toes. As a rule, the colour of the skin is greenish olive, with yellow or 
orange spots, passing into streaks on the under surface of the head; while some 
species have a few much larger eye-like spots on the back of the shell. 
Although the whole of the soft-tortoises are included in a single 
family, they are arranged in six distinct genera, three of which are 
nearly allied to one another, as are likewise the remaining three among themselves. 
The first and largest genus, Trionyx, contains fifteen living species, with a distribu¬ 
tion coextensive with that of the family. These are characterised by the absence 
of a fold of skin on the hinder part of the under shell, beneath which the leg may 
be concealed, by the sculpture on the shell being generally in the form of wavy 
raised lines, and by the hyo- and hypoplastral bones of the lower shell remaining 
distinct from one another. In the skull, as shown in the figure on p. 89, the 
sockets of the eyes are placed relatively far back, and widely separated from the 
aperture of the nose. Among the better-known species we may mention the 
Gangetic soft-tortoise ( Tgangeticus), now confined to the river system from which 
it takes its name, but formerly found, as shown by fossil specimens, in the 
Narbada; the length of the shell and fleshy disc reaching as much as 2 feet. 
Like all the Old World representatives of the genus, this species has eight pairs of 
costal bones in the carapace; while it belongs to a subgroup characterised by 
having two neural bones between the first pair of costals, and by the absence of a 
pronounced ridge in the middle of the upper surface of the extremity of the lower 
jaw. The soft-tortoise of the Nile (T. triunguis), ranging over Africa and Syria, 
and attaining still larger dimensions, belongs to a second subgroup, distinguished 
by having only a single neural bone between the first costals; while Phayre’s soft- 
tortoise (T. phayrei), of Burma, may be taken to represent a third section differing 
from the last by the presence of a median ridge in the front of the lower jaw. On 
Typical Genus. 
