354 AMERICAN TESTUDINATA. Part II. 



arrangement of which vary somewhat in diflerent genera. The skin of the head, 

 neck, hmbs, and tail, is all more or less covered with scales, and where the sur- 

 face is exposed, when the limbs are retracted, or when the animal is walking, the 

 scales are imbricated, and form a continuous covering. 



The habitat varies widely in this family. Nearly all live more or less in 

 the water, in marshes and pools, or along the edges of ponds and still streams; 

 but one genus, at least, never seeks the water, and with those that do, the pro- 

 portion of life passed in that element varies exceedingly; indeed, the family pre- 

 sents a gradual series, from those which are almost exclusively aquatic to those 

 which live always on land. In swimming, the feet and legs move in a plane 

 nearly parallel to that in which the body is moving, that is, horizontal, if the 

 animal is moving horizontally. In walking, also, the humerus and femur move 

 nearly horizontally, which is made necessary by the great width of the plastron 

 under them ; but at the elbows and knees, which move around or beyond the 

 edge of the plastron, the legs are turned down to an angle, greater or less, 

 according as the body is raised to a greater or less height from the ground; but 

 the knee, even when brought flirthest forward, is never opened to a right angle, 

 as it is in the Testudinina, and the body is not raised up upon the ends of the 

 toes of the fore feet, but the whole foot of both pairs is brought to the ground. 

 Thus the body is not carried so high as in the Testudinina, and the gait is 

 much less firm and steady. When molested, these Turtles resort to the nearest 

 hiding-place ; the aquatic species, if near the water, seek that as the first shel- 

 ter ; if hindered in this, they withdraw the head, limbs, and tail into the shield, 

 and, if pressed still further, they stretch out the head and bite. When they retreat 

 within the shield, the head is carried far back between the shoulders, and the neck 

 drawn in naked among the viscera; the legs are folded between the inner parts of 

 the projecting free edges of the shield, and the tail is turned to one side. 



The knees and elbows do not, as in the Cinosternoida?, slip in naked among 

 the viscera, but the skin keeps its position close around them. The humerus is 

 carried round before, and almost directly across, the front end of the body, but a 

 little raised at the elbow ; the forearm is turned back upon the humerus, and 

 the foot upon the shoulder, the toes reaching to the shield where the edges of 

 the carapace and plastron meet. See PI. 1, 2, 3, 4, and 5. The elbows do not 

 come together, but leave room between them for the passage of the head. The 

 head is often placed between the elbows, but sometimes drawn further back ; in the 

 latter case, the skin folds together before it. The femur is carried round by the 

 side of the pelvic region, so as to reach almost directly forward, but a little upward ; 

 the foreleg is turned back upon the femur, and the foot so turned forward that 

 the inner edge rests upon the foreleg. When the limbs are in this position, the 



