THE SOFT TORTOISES 2457 



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; hav- 

 ing, 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 characterized 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 their long necks, which, together with the head, can be completely withdrawn into 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 color 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 

 18 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 characterized 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 hyoplastral and hypoplastral bones of the lower shell remaining 

 distinct from one another. In the skull, as shown in the figure on p. 2446, 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 ( T. gangeticus] , 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 two 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 characterized 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 (7! 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 

 the other hand, all the American soft tortoises, of which T. ferox is a well-known 

 example, differ by having only seven pairs of costal bones. Numerous representa- 



