334 AMERICAN TESTUDINATA. Part IL 



inner toes, but the -whole foot aids in removing it. In wallving on dry land, the 

 legs move as nearly horizontally in propelling the body forward as is consistent 

 with the resistance offered by the ground. The animal readily resorts to the shield 

 for protection. The neck and head are withdrawn entirely within the sliield, the 

 skin rolUng off from the greater part of the neck, and allowing it to protrude 

 naked among the viscera. The legs are withdrawn horizontally, and the skin slips 

 off so far that it does not surround them, except below the knees and elbows. 

 When thus withdrawn, the humerus is carried round into or before the wide spread 

 scapular arch, the elbow being placed very near the head or neck ; thfe fore leg 

 and foot are turned back upon the humerus, the flat surface of the foot being 

 nearly horizontal, so that its outer edge rests against the humerus. The knee is 

 carried almost directly forward, the fore leg turned backward against the femur, 

 and the foot again turned somewhat forward, its flat surface being nearly hori- 

 zontal. See PI. 6. 



It is easy to perceive the close relations which exist, in this family, between 

 the mode of locomotion, the movements and position of the limbs, and the general 

 form of the body. The limbs, for example, move and are withdrawn horizontally ; 

 so also is the body widely stretched out horizontally, and moreover it is flat and 

 low. The flat front end offers little resistance to the water before it; its sharp 

 outer edge offers as little resistance also to the water which is driven back by 

 the fore feet. Again, this low end is well adapted to entering the mud, and the 

 fore feet to loosen and remove as much of it as is necessary to enable them to 

 bury themselves in the soft ground. The flattening of the carapace backward is 

 necessary to allow free horizontal movement to the hind legs. 



The habits of the Trionychidae are little known. In confinement, they exhibit 

 great quickness in their motions, which are abrupt and unsteady, except when 

 they swim rapidly in one direction. They then dart their long, slender neck 

 quickly forward or sideways and upwards, as tlie Snakes do, and bite in the same 

 way, striking suddenly the objects they aim at. Different attitudes of the North 

 American species are represented in PI. 6. They feed upon shells, especially upon 

 Anodontas and Paludinas, fragments of which I have frequently found among their 

 fjBces and in their intestine. They probably grope for them in the mud with 

 their proboscis. They lay from twelve to twenty and more eggs, of a spherical 

 form, and about the size of a musket ball, which they deposit on shore in the 

 sand near the water's edge. The shell of these eggs is thick but very brittle. 

 The eggs of the Trionychidas and those of the Cinosternoidge are the only 

 Turtles' eggs I know, the shell of which is not more or less flexible. 



