Chap. III. GENERA OF CINOSTERNOIDiE. 421 



and descend less helow the carapace ; the fourth pair of hony plates is nar- 

 rower at its front end, and narrows continually thence backward, its sides being 

 straight, and not curved outward, as in the first group ; and the sutures of the 

 first and fourth pairs, with the second and third, are but slightly movable in the 

 adult, and in some cases not at all so. Thus the spaces around the free edges 

 of tlie plastron are here larger than in the first group; and besides, the protection 

 from the shield is still less on account of the slight movability of the parts of 

 the plastron upon one another. There are, besides, certain other tendencies that 

 become important in connection with their constant characters. In Cinosternoids 

 the tendency is to a more regularly arched carapace ; in Ozothecoids, to a sharp 

 ridge along the l)ack, the sides spreading wide apart downward, so that the body 

 is generally broader between the outer edges, but less deep below them, than in 

 the first group. The scales on the plastron of the Cinosternoids are well devel- 

 oped and well defined, and cover its whole surface ; but in the Ozothecoids they 

 are more irregular, and often separated by large, scaleless spaces between them ; 

 and the fourth pair of bony plates reaches forward on to the third pair, which 

 is never the case in the Cinosternoids, for there it would interfere with the motion 

 of the hinge. The scales of the shield differ also ; in Ozothecoids they have 

 a marked tendency to overlap those farther back, the centre of growth receding 

 gradually backward of the centre of figure, as in the Chelonioids, and some exhibit 

 even distinct traces of imbrication. In both groups there are two or more horny 

 papillae imder the chin. The principal differences between these groups all go 

 to bring the body more under the protection of the shield in Cinosternoids than 

 in Ozothecoids, and to give the legs freer motion in the latter than in the former. 

 These characters are easily traced to corresponding habits of these animals ; for. at 

 least as far as we are acquainted with the members of these groups, the Cinoster- 

 noids resort, in danger, more to the shield, the Ozothecoids, to flight; the former 

 live more on land, the latter more in deep water, and are also the more shy, 

 and the (juicker in their motions. These characters, thus connected with the 

 general form, and impressing npon it such decided tendencies, are clearly sub- 

 family characters, and the gi-oups themselves are sub-families. 



Within tlie limits of each of these sub-families of Cinosternoids, minor group-s 

 containing one or more species, may l)e distinguislied, that dilll-r in tlie structure of 

 the jaws and the parts dependent upon them, in the way of taking food, and, to 

 some extent, in the kind of food sought ; in short, in the voluntary organs of 

 nutrition, and the parts concerned in it. At first sight, these groups, based on 

 one set of organ.s, may seem arbitrary; but if it is remembered to what extent 

 the acts of animals are directed to getting food, how far their sensations are 

 gratified by this act, and how largely their instincts are concerned in it, it will 



