308 AMERICAN TESTUDINATA. Part II. 



SECTION XVIII. 



SUB-ORDERS OF TESTUDLSATA. 



The Suh-Order of Sea-Tiuiks — Chelonii, Opp} The sea is the home of these ani- 

 mals. They sAvim freely, and sustain themselves in the water for any length of 

 time without seeking the bottom or the shore for support or rest. They never go 

 on land, except to lay their eggs, and then proceed only a short distance from the 

 shore, moving slowly and in a very constrained way. They swim almost entirely 

 by means of the front limbs; the other pair act independently, and are chiefly 

 useful in aiding to balance the body and guide the general course. The fore- 

 arm and hand form a sort of paddle, or rather a wing. These two wings are 

 raised together, and also strike downward simultaneously; but the blow is not exactly 

 vertical, the wings being carried forward as they rise, and approaching the breast 

 when brought down. They descend fiirther below the body than they rise above 

 it, and their motion is very similar to that of a Bird's -\vings ; indeed, the animal 

 may be said to fly through the water. On land, these animals still move the 

 front limbs together, carrying them forward, throwing the weight of the body upon 

 the elbows or thereabouts, and then pulling the whole toward them. 



The peculiar flying locomotion of this sub-order affects the general symmetry 

 of the body very essentially in two ways : first, it makes it necessary that the 

 bulk of the body should be carried forward near the wings, otherwise the animal 

 could not control it ; secondly, the force necessary to propel the wings requires 

 a large muscular apparatus, and this takes much room, so that the fore part of 

 the body (dividing the whole crosswise into two parts of equal lengths) far outr 

 weighs the hind part, being in bulk in the proportion of two to one ; the fore part 

 is broad and high, the hind part descending and narrowing gradually. The humerus 

 is very short, and the extensive surface of the wing arises principally from the 

 blade, which is formed of the forearm and hand. This blade is long, broad, and 

 thick at the base, thin along the inner margin, and pointed at the outer end ; 

 it is turned back at the elbow, and cannot be brought out in a line with the 

 humerus, though it is capable of moving towards it and away from it through a 

 long arc. The force and general direction of the blow is given by the muscles 

 of the shoulder; but the surface presented is determined in a great measure by 

 the rotation at the elbow, at the wrist, and within the hand, the blade being 



^ This sub-order was first recognized and char- 309. Compare also Sect. 2, p. 241, where the syno- 

 acterized by Oppel, in the work quoted below, p. nymes of the sub-orders are given, and PI. I.-VI. 



