CiiAP. I. SUB-ORDERS. 309 



turned, now edgewise, now flatwise, to the resisting medium. The fore-arm is short ; 

 the radius is carried down and hack under the ulna, and the inner side of the 

 hand carried down with the radius. By this pecuhar arrangement, the flat surface 

 of the hand is more directly presented to the resistance of the water in the 

 downward and hackward flying blow. The fingers add the greater part to the 

 length of the l)la(k' ; they are very long, stiff, and fixed in their respective places, 

 tlieir only movement consisting in a slight accommodation to the turning of the 

 whole blade. Tlie muscles and skin form one continuous surfiice over the fingers, 

 excepting the last joint of one or two of them, Avhich, sometimes at least, pro- 

 trudes, and has its protruding surface covered with a nail. The coracoid process is 

 very long; the other bones of the shoulder apparatus short and stout. It is 

 necessary to the flying locomotion of this sub-order that the wings should have a 

 free sweep by the front end of the body, and that nothing should hinder them 

 in rising and descending or moving backward and forward ; hence the shield 

 cannot project forward above or below, and the humerus carries the elbow, in all 

 its positions, beyond it. Again, as the humerus is so short, and the blade so 

 long, the front limbs cannot be brought round before the body ; but, when at rest, 

 the blades hang down, or are placed beside or upon the outer edges. Although 

 the front Hmbs are the principal locomotive organs, and are essentially wings 

 in all their operations, there is yet one marked structural difference between 

 them and the wings of a Bird ; for with the Turtles the humerus i-eaches 

 forward, and the forearm and hand are turned backward in one line from the 

 elI)ow, whereas with the Bird, the humerus reaches backwai'd, the forearm for- 

 ward, and the hand again backward, the main surface of the wing being in the 

 angle of the forearm and hand, instead of being, as in Turtles, in the angle of 

 the luunerus and the limb below. The pelvis and hind legs are very small. The 

 legs, as was said above, do not move together with the wings, and they take little 

 part in locomotion be^'ond aiding to balance and guide the body. The femur 

 and leg are short, and the toes also short, compared with the fingers, but they 

 form the greater part of the whole blade below the knee. The leg and foot 

 are formed into a paddle, much smaller than the blade of the front limb.s, and 

 broade.st near the outer end. Below the knee, this blade is generally turned back- 

 wards ; but it moves through a long arc back and forth, and may even be brought 

 out upon a line, or nearly on a line, with the femur. The paddles often strike 

 directly downwards, so that the plastron cannot extend under them, and is very 

 small under the pelvis. 



The neck is short and little fie.vible, so that the head is not withdrawn under 

 the carapace ; instead of this, it is protectetl by a very large development of the 

 postrfrontal, parietal, jugal, and mastoid bones, making a bony arch over the whole 



