428 CHELONIA. 



legs, properly so called. There are sometimes two claws to either 

 flipper, at others only one, or else none at all. Whenever present, 

 the thumb nail is more developed in the male sex than in the female. 

 The tail is always short, thickish, subconical, and tapering. 



The head is subquadrangular across the orbits, and more or less 

 rounded, abbreviated, or subconical anteriorly, covered with polygonal 

 plates, except in the adult of the "Leather Turtle" {S2:)hargis), in 

 which the skin of that region is smooth and exposed as elsewhere. 

 The orbits themselves are large. The nasal cavity rather small, and 

 wider than long. The tympanum is hidden under the temporal plates. 

 The neck is but little flexible : hence, the head is not retractile under 

 the carapax. 



Their food consists chiefly of marine plants, some species feeding 

 likewise on Crustacea and molluscs. Essentially of marine habits, 

 they never leave the water, except at the period of laying eggs, which 

 they deposit in the sand, not far from the shore. 



This group includes the largest species of the order, and with whom 

 the crocodiles alone, amongst other reptiles, can be compared as to 

 size. The flesh of some of them is served upon our tables, as well as 

 their eggs, which constitute an article of luxury. Others aflbrd to 

 commerce their " shells," which is used for various economical pur- 

 poses, hence, of great utility to man, as well as a source of consider- 

 able revenue. 



Syn. — Chelonii, Oppel, Kept. Prodr. 1811, 4, 6, & 8. — Gray, in Ann. of Philos. X, 

 1825,210) &, Sjnops. Kept, in Gnf. Anim. Kingd. IX, 1831, 3.— Wiegm. & Ruthe, 

 Handb. Zool. 1832, 163.— Bonap. Sagg. Distr. Anim. Vertebr. 1831, 70; &, Chelon. 

 Tabul. Anal. 1836, 3.— Agass. Contr. nat. Hist. U. S. Amer. I, 1857, 249 & 308. 



Pinnata, Merr. Tent. Syst. Amph. 1820, 17.— Bell, in Zool. Journ. Ill, 1828, 

 516. 



Carettoidea, FiTZ. Neue Class. Kept. 1826, 5. 



Uretmochelones, Ritgen, in Nov. Act. Acad. nat. cur. XIY, 1828, 269. 



Olacopoda, Wagl. Naturl. Syst. Amph. 1830, 133.— FiTZ. Syst. Kept, i, 1843, 30. 



Thalasdtes, DuM. & BiBR. Erpet. g^n. II, 1835, 506. 



Thalassidae, Cantor, Catal. Rept. Malay. Pen. 1847, 11. 



Sea Tortoises or Turtles, Shaw, Gen. Zool. Ill, I, 1802. 



Observ. — Two families constitute this sub-order, the SpJiargidae, or 

 Leather Turtles, and the Ghelonidae, or Scaled Tortoises : of the latter 

 alone, do we find representatives in the collection made by the U. S. 

 Exploring Expedition. 



