Chap. II. THE CIIELONIOID^. 325 



distinct family, the limits of the Chelonioidas are again circumscribed, as they 

 were at first. 



The form of the Chelonioida) is that of a heart llattened on one side, from 

 the broad end of which projects a large head upon a thick neck, and from the 

 widening side of wliich protrude, in front, a pair of large, fiat, wing-like, scaly 

 flappers, and below the narrow part of which hang another pair of broad, short, 

 scaly rudders. As illustrations of the prominent features of this fomily, see sev- 

 eral attitudes of the Lojigerhead Tui'tle in PL 6. 



The body is not, as in Sphargididse, broadest about the arch of the second pair 

 of ril)s, where the carapace and plastron first meet to encircle it, but continues 

 to widen from the front end to about midwa}', and thence narrows to a point 

 behind ; while the vertebral column descends constantly and gently along the whole 

 thoracic, abdominal, and pelvic regions to the tail. The carapace is a roof 

 slanting doAvn on either side from the vertebral column, and thus it continues over 

 the pelvis as well as along the thoracic and abdominal regions, and terminates 

 behind the sacrum, by the meeting at a point of the outer edges and the middle 

 line ; the only deviation of its outline in passing from the abdominal to the pel- 

 vic region being a slight elevation of the lower edge above the hind legs. The 

 carapace is bordered all round by a distinct marginal rim ; about the front end 

 this rim is turned downwards, but shortly behind the beginning of the union with 

 the plastron it flares outward, and so continues to the hind end. In consequence 

 of this peculiar form of the marginal rim, the shoulders are much more protected 

 than in the Sphargidida? ; its width adds still more to the protection of the hind 

 limbs. The plastron is joined to the carapace from near the arch of the second to 

 between the arches of the sixth and seventh pairs of ribs. The plastron and the 

 carapace meet at a sharp angle, the plastron descending but little below the level 

 of the outer edges. The plastron, like the carapace, grows broad to about midway 

 of the body, and narrows thence backward ; it underlies a very large part of the 

 lower surface. The opening about its hind end, for the protrusion of the limbs 

 and tail, is smaller and more under the body than in the Sphargidid*. Thus the 

 shield, — instead of having, as in Sphargididaj, a conical form wrapped closely around 

 the thorax and abdomen, and growing narrow backward in passijig over those 

 regions, then narrowing still much faster to pass over the pelvis, — presents here an 

 extended roof-like carapace, with the outer edge sharply defined, flattened upon the 

 sides, broadest about midway, - protecting above the whole body from one end to 

 the other, and a plastron which descends but little below the outer edges. 



The shield, having a form widely diifereut from that of the Shargidida\ needs 

 also a different structure and different means of support. Instead of a con- 

 tinuous layer of fibrous corium protected above by a thin bony sheet, we have 



