THALASSOCHELYS. 429 



Fam. CHELONIDAE, Bonap. 



The Chelonidae may be distinguished from the Spliargidae at the 

 very first glance by the existence of large epidermic scales covering 

 the carapax and sternum, and by the presence also of horny plates 

 over the head. In Sphargis, cephalic plates are observed only in 

 young specimens ; in the old, the head exhibiting the naked skin, as 

 well as the surface of the limbs, Avhich in Chelonidae are protected by 

 scales and plates of various size and shape. The apex of the lower 

 jaw in Sphargis is acerated and curved upwards, whilst the upper jaw 

 is notched laterally as well as anteriorly. In CheJonidae the shape 

 and structure of the jaws is peculiar in each genus. It may be that 

 the same would be the case amongst Sphargidae were there more than 

 one genus composing it. 



^Y^.— Chelontadae, Gray, in Ann. of Philos. X, 1825, 212; Catal. Tort. Croc. & 

 Amphisb. Brit. Mus. 1844, 51 ; &, Catal. Shield. Kept. Brit. Mus. 1855, 70.— Bell, in 

 Zool. Journ. Ill, 1828, 516. 



Chelonidae, Bonap. Sagg. Distr. anim. vertebr. 1831, 71; &, Chelon. Tabul. anal. 

 1836, 4 & 9. 



Chehnioidae, Agass. Contr. Nat. Hist. U. S. Amer. I, 1857, 324. 



Cheloniae, Wiegm. & Ruthe, Handb. Zool. 1832, 165.— Fixz. Syst. Kept, i, 1843, 

 30. 



Observ. — Whilst the family of Sphargidae is composed of but one 

 genus and a single well-determined species, that of Chelonidae em- 

 braces various genera, some of which being composed of several species. 



Genus THALASSOCHELYS, Fitz. 



Gen. Char. — Head very large; jaws robust, anteriorly compressed, 

 sharp and even upon their margin, and curved towards one another 

 at the tip. Two pairs of frontal plates ; an interfrontal, sometimes 

 divided ; a vertex plate ; and, two pairs of parietals. A middle 

 occipital, very large ; two pairs of latero-occipitals ; and several post- 

 occipitals. Three postoculars. Mental shields present. Side of 

 lower jaw protected by angular plates. Carapax cordate, ovate, 

 posteriorly indentated upon its periphery, covered with fifteen un- 



108 



