342 AMERICAN TESTUDINATA. Part II. 



the outer edge descends steeply from the front margin to about midway, and 

 rises from thence backward, but less steeply. Thus the upper surface is a shed- 

 roof falling backwards, and curved down on either side, lowest abovit the middle, less 

 and less toward the ends. The arch from side to side is somewhat flattened on 

 the top for nearly the whole length of the back. The base, or flattened part of 

 the lower surface, upon which the body rests, is very small ; it is but little below 

 tlie lowest j)art of the outer edge ; it extends lengthwise from near the front end 

 of the body under the whole dorsal vertebral column and a part of the sacrum, 

 not reaching the hind end of the body ; it is widest aliout midway, where it 

 includes between a third and a half of the width of the lower surface ; from 

 thence it narrows to a point behind, and to a blunt but narrow end in front. 

 Thus the space around it, that is, between it and the outer edge of the body, is 

 very broad, including the greater part of the whole lower surface ; it is high and 

 steep in front, lower and more horizontal behind. The carapace projects beyond 

 the attached surface of the body all round, except where it passes over the neck, 

 and where it is joined to the plastron. At the suture with the plastron it is 

 turned somewhat down. 



The plastron is fixed, on either side, to the outer edge of the carapace where 

 it descends the lowest, about midway between the front and hind ends, from the 

 arch of the fourth to that of the sixth pair of ribs, sometimes extending a little 

 beyond, and sometimes not quite reaching, these bounds ; from thence inward it 

 descends a little, and narrows very fast toward the base, or flattened part of the 

 lower surface, where it lengthens again much faster, and spreads out under the whole 

 of that surface, and as the free edges do not project, they take its form and size. 

 Thus the whole plastron is small. The bridge which passes from its lower flattened 

 part to the carapace is extremely narrow; the openings in the shield for the 

 protrusion of the head and limbs at the ends of the body are large, including much 

 the larger part of the whole lower siu'face ; the front opening is high and exposed, 

 and the hind one low under the body, and protected ; these two openings are 

 separated from one another on each side only by a narrow isthmus. 



The shield in the adult is completely ossified, and the bony derm is regularly 

 divided into plates, and more intimately connected with the true skeleton than in 

 the Trionychidae. In the carapace, the eight costal plates, the vertebral row, and 

 the marginal rim, are constant. The vertebral row is continuous from one end of 

 the carapace to the other ; it consists of twelve jjlates in all, eight of which corre- 

 spond to the costals, and lie between them, being fixed to the vertebrae below; 

 one reaches from the first of these forward between the first pair of costals into 

 the marginal rim, terminating it in front ; three more carry the row back to its 

 hind end, the last one entering into the marginal rim, and terminating it behind. 



