394 REPTILIA ORDER ILCHELONIA. 



A fresh-water turtle from one of the Papuan rivers described under the 

 name of Carettochelys, resembles the true marine turtles in having the limbs 

 modified into flippers, although displaying all the other char- 

 Family acteristics of the present sub-order. Only two claws occur 

 Carettochdyidce. on each flipper ; and the shell is remarkable for having a 

 continuous leathery skin in place of the usual horny shields. 

 There are certain other structural features in this little-known and rare 

 Cheloriian by which it differs from its allies. 



Sub-Order III. Trionychidce. 



The third and last sub-order of the Chelonia includes only the soft tortoises, 

 represented by six genera, all of which are comprised in the single family 

 Trionychidce. All these tortoises have very long necks, which can be com- 

 pletely withdrawn within the margins of the shell by an S-like flexure in the 

 same manner as in the Cryptodira. The jaws are hidden by the fleshy lips, 

 and the muzzle is prolonged into a short proboscis. The feet are fully 

 webbed, and only the three outermost toes are furnished with claws a 

 peculiarity from which the scientific name of the group is derived. None of 

 these features are, however, of sufficient importance to justify the separation 

 of the group as a distinct sub-order; but the shell is very peculiar. In place 

 of the usual horny shields, both the upper and lower shells are invested with 

 a thin, continuous leathery skin, through which can be seen and felt the 

 raised sculpture ornamenting the surface of the underlying bones. Marginal 

 bones, if developed at all, are restricted to the hinder border of the upper 

 shell, and elsewhere the margins of this shell are continued into a wide, soft 

 expansion of skin, so that the shell itself only occupies the region of the back. 

 There is no attachment between the upper and lower shells, and the latter is 

 only very imperfectly ossified. A greenish olive ground-colour, upon which 

 are yellow or orange spots, is the general type of coloration, the spots being 

 replaced by streaks on the under surface of the head. At the present day 

 the soft tortoises are restricted to the fresh waters of the warmer regions of 

 North America, Asia, and Africa. In habits, these very peculiar Chelonians are 

 purely aquatic, and although the majority are confined to fresh waters, some fre- 

 quent estuaries. They are all carnivorous ; and, being extremely bold and fierce, 

 are highly dangerous to bathers. All are mainly nocturnal, and when basking 

 on a mud-bank, or on shore, for the purpose of laying their eggs, extend their 

 long necks after any sudden surprise in a peculiarly cautious and stealthy man- 

 ner. The spherical eggs are buried in the mud by the female as soon as laid. 



The typical, and at the same time the widest spread, genus is Trionyx, which 

 has a geographical distribution as extensive as that of the family, and is the 

 only one found in America, fifteen distinct species being known. In this 

 genus, the sculpture on the surface of the shell usually takes the form of 

 irregular wavy ridges; while in the lower shell the hyoplastral and hypo- 

 plastral bones of each side are separate. A distinctive feature is the want of 

 a fold of skin on the hinder part of the plastron, which in some of the allied 

 forms conceals the hind-leg. The head is relatively short and broad, with 

 the eyes situated far back. The shell and leathery disc of some of the members 

 of this genus not unf requently measure as much as a couple of feet in length, 

 and the species are arranged in groups according to the number of bones 

 entering into the composition of the upper shell, and the absence or presence 



