310 AMERICAN TESTUDINATA. Part II. 



head back of the eyes, and projecting somewhat over the neck, entirely covering 

 the temporal muscles above. Thus neither head nor limbs can be withdrawn 

 into the shield, and the front limbs cannot even be brought round before the 

 body, but they can all be drawn back somewhat. So the method of protect- 

 ing the extremities and the head, which is so fully developed in the other sub- 

 order, and is so characteristic of the order, is here but just begun. The shield 

 itself is here much less developed than in the other sub-order. In one family, 

 the Sphargididse, it is little more than a broad girdle, encirchng the thorax and 

 abdomen ; its bony part does not rest upon the ribs, and has no marginal 

 rim. In the other family, the ChelonioidoB, the shield is somewhat larger, cover- 

 ing the pelvic region above; but still the front limbs, including the shoulders, are 

 free and exposed, and so also are the hind limbs below, including the hips. 

 Although the bony derm rests upon the ribs, their union never becomes so inti- 

 mate as in the other sub-order, and the plastron is but imperfectly ossified and 

 rather loosely connected with the carapace. Thus we find the most prominent 

 characteristic features of the order least developed in this sub-order; and if we 

 add to this the habitat, the mode of locomotion, the paddle-like structure of the 

 limbs, the reduced state of the hind pair, the want of specialization in the neck 

 vertebrae, and the unsymmetrical relations of the two ends of the body, we can- 

 not hesitate to consider this group as the lowest of the Turtles, and to recog- 

 nize a kind of gradation in rank between them and the Amydas. But here, 

 in this lowest group, where the characters of the order are least prominent, we 

 find features of form and structure which remind us of animals higher in the 

 series, and belonging to another class. The mode of locomotion, the fonii and 

 structure of the locomotive apparatus, the great preponderance of the fore part 

 of the body, the bill-hke jaws, the overlapping of the scales in some, as in 

 Penguins, are .all characters which belong to the class of Birds, and are there 

 only carried out to their fullest development. 



The Snh-Onler of FreslMvatcr and Land Turtles — Amyd.e, Opp} The habitat is 

 various. Some species spend nearly all their life in the water, some live partly 

 under water and partly on dry land, and some entirely on dry land; yet none 

 are entirely aquatic, none remain for any great length of time in the water with- 

 out seeking the bottom, nor can they swim unsupported for a long distance. When 

 in the water, they remain usually at the bottom, either at rest or moving along 

 over it. They seldom swim freely, except when they rise to the surface or descend 

 to the bottom. So, in fact, they dwell principally upon land, sometimes under the 



^ Like the sub-order of Chelonii, that of Amydte pel, in liis cLassical paper, Die Ordnungen, Familien, 

 also was first recognized and characterized by Op- u. Gattungen der Reptilien, Miiiichen, 1811, 1 vol. 4to. 



