Chap. I. SUB-ORDERS. 311 



water, and sometimes in the open air. The difierence between these two conditions 

 does not acquire much importance with reference to tlie characters of the sub-order, 

 as will be seen from the foct that, in the family of Emydoidai, one genus at least 

 never goes into the water, while several genera live the greater part of the time in 

 water, and there is a series of intermediates. These differences affect the structure 

 and symmetry in a smaller degree, and are not to be compared in importance with 

 those which distinguish the sub-orders; they do not essentially alter the mode of 

 progression. 



The locomotion is entirely different from that of the sea Turtles. It no longer 

 takes place by a flying, but by a walking motion ; the weight is not thrown 

 upon the front limbs, but is almost equally supported by both pairs ; the front pair 

 are not carried up together, and then brought doAvn simultaneouslj', but they alter- 

 nate with one another, as do also the hind pair ; the front legs of one side 

 move with the hind legs of the other side, so that the two pairs act in concert ; 

 further, they move back and forth below the carapace, in a diagonal plane 

 between the perpendicular and the horizontal diameter of the body. The two 

 pairs are nearly equally developed, as also are the pelvis, and shoulder apparatus. 

 As the bulk of the body is no longer thrown upon the front limbs, and as the 

 muscular apparatus of the two pairs occupies about equal space, there is no such 

 contrast between the two ends of the body as exists in sea Turtles. This 

 mode of progression, and the consequent symmetry, allow greater development of 

 the bony shield than can take place with the other sub-order. As the fore limbs 

 are not raised high up, when moving, the carapace may be extended forward 

 without interfering, and as they are not brought far down crosswise over the body 

 toward one another, the plastron may be broad between them. The carapace is 

 always broad above the pelvis, and covers all that part of the body, and the hind 

 legs, when they are at rest; the plastron is sometimes broad under the pelvis 

 and the hind legs. 



The limbs are never reduced to paddles or wings ; the feet are alwavs distinct 

 from the legs; the articulations at the wrist and ankle joints allow distinct move- 

 ments, and not merely a kind of yielding to the turning of the whole limb, below 

 the elbow, as with the sea Turtles. In the feet of this sub-order, the toes never 

 have the great length which distinguishes them in the wings and paddles of sea 

 Turtles. When the foot is adapted to walking on dry laud, the toes are shortr 

 ened, and the whole concentrated, and their joint with the leg above is rather 

 stiffened ; when it is more adapted to swimming, there is greater freedom of 

 motion at the wrists and ankles, and between all the bones of the feet below; 

 the phalanges are prolonged, and the toes joined by a broad web, capable of 

 beuig spread far apart and closed together. When the blow is struck, a broa^. 



