Chap. II. THE TRIONYCIIID.E. 333 



The free skin is loose about the neck and Hnibs. There are no epidermal 

 scales, excepting a few narrow, long ones on the limb.s, which serve not so much 

 for protection as to stiflen the web. 



The principal habitat of the members of this family is the muddy bottom of 

 shallow waters. They bury themselves in the soft mud, leaving only the head, 

 or a small part of it, exposed. They take breath from time to time, without 

 moving the body, by raising up the long neck and head and carrying the leath- 

 ery snout above water. They sometimes stay under water a long time, Avithout 

 taking breath ; in one instance, a specimen has been seen to remain mider water for 

 more than half an hour without raising its snout above the surface. The nature 

 of the habitat is clearly connected with some of the prominent family character- 

 istics. For instance, the ])uried bod}^ needs not the protection of the fully ossified 

 shield which the other families have : the long neck and head, the projecting snout, 

 and the free communication between the nasal openings and the mouth are all con- 

 nected with the manner of taking breath. These animals rarely go on dry land, 

 and when they do, their locomotion is laborious and constrained ; yet it is iden- 

 tical with that of the other AmydjB in the alternation of the limbs of the two 

 sides of the body. When moving through the water, they strike horizontally Avith 

 both pairs of limbs,^ alternating however between the right and left foot of each 

 pair ; but when they start suddenly, the front limbs are seen moving together 

 towards the tip of the snout, and then striking simultaneously backward with great 

 power to propel the body forward. iVs the shoulders are placed so near the edge 

 of the body, and the shield does not project free about the front end, the front 

 limbs move mostly beyond the shield, in front and at the sides ; and as the 

 outer edge is sharp, and the feet are broad, their web reaches above as well as 

 below the plane of that edge, and when they strike, they drive the water back, 

 partly over and partly under it. The hind legs move back and forth below the 

 carapace and drive the water backward without hinderance, for the flexible broad 

 rim is so light in the water that it yields readily to the current. When these 

 animals move along on the bottom, the limbs still move horizontally, the web 

 striking against the water, and the inner toes against the bottom. They also bur- 

 row horizontally, going under the mud only to the depth of a thin layei-. When 

 burrowing, they carry the hind feet forward and outward, and thus bracing them- 

 selves, press the body forward, digging a part of the mud with the fore feet, 

 and raising a part of it up on the body ; the mud is looseneil by the strong 



' All tlie figures which I know, representing below the level of llio lower sui-fuce of the body, 

 members of this family, are very incorrect. The as they are represented in all tlu' figures of Tri- 

 feet are never brought down, as in other Amyda^, onychida* thus far published. 



