312 AMERICAN TESTUDINATA. Part II. 



webbed surface is presented to the water, and when the foot is witlidrawn for 

 another blow, the web is folded ; — a very different way of controlling the surface 

 presented to the resistance of the water from the turning of a stiff blade, now 

 edgewise, now flatwise, which takes place with the sea Turtles. The limbs, thus 

 jointed and proportioned, can always be withdrawn under the carapace, the front 

 pair before, and the hind pair behind, the main bulk of the body ; the neck 

 is always retractile enough to allow the head to be withdrawn partially, and 

 generally completely, within the shield ; and we nowhere find the temporal muscles 

 protected by such a very broad bony arch as exists in the sea Turtles. 



Here, then, those features which are most peculiarly characteristic of the order 

 of Turtles, namely, the protection of the body by the shield, and the withdrawing 

 into the shield of the head and neck, and limbs and tail, are most fully devel- 

 oped. This sub-order occupies clearly a higher rank than the other; the equilibrium 

 of the body, the higher development of the limbs, the cooperation of both pairs 

 in the progression, the greater S2oecialization of the neck vertebrae allowing the 

 head to be withdrawn under the carapace, the nature of the habitat, and the 

 higher degree to which the characters of the order are carried, — all these features 

 assign to the Amydae a rank superior to that of the sea Turtles. In this higher 

 group, the Bird characters, which are so prominent in the sea Turtles, yield to 

 the characters of a higher class. The equal development of the two pairs of 

 limbs, their full cooperation, the Avalking locomotion, the elevation of the body 

 free from the ground while walking which takes place with most of them, and 

 the general symmetry of the body, are characters which remind us of the class 

 of Mammals. And the analogy is the more striking when we remember that this 

 is the first instance, in the series of Vertebrata, of real walking, unless the running 

 of some toads be considered as such; for the Salamanders, the Lizards, and the Croc- 

 odiles move partly by means of the vertebral column bending and carrying the 

 legs forward, now on one side and now on the other. These Mammalian charac- 

 ters may be not so striking here as the Bird characters are with the other group, 

 for the class of Reptiles is further removed from that of Mammals than the 

 Birds; stUl the analogy is too complete and too clear to be accidental, or to be 

 passed over in silence. One marked difference between the locomotion of these 

 Turtles and that of Mammals is, that in the former the knee and elbow joints 

 open in the same direction, whereas in Mammals they bend in directions opposite 

 to one another. 



The characters of the Chelonii and Amydje show these two groups to be 

 sub-orders, and neither families nor orders proper, as they partake of the features 

 of orders, without extending to the whole structure of all the different systems 

 of their organization. 



