CuA?. I. THE SKIN. 257 



skeleton. Thn.s it appears tliat in Spharj^is the trnnk is inclosed in a clennal 

 bony girdle which is circumscribed in front, behind, and on the two sides; under 

 this solid envelope follows a coarse felt of soft corium without lime deposits, and 

 under this finally lies the true .skeleton. In Sphargi.s, the ossifications of the skin 

 have thus least to do with the skeleton proper, while the connection of the dermal 

 and the true skeleton is carried furthest in land Turtles. We may say, therefore, 

 that if the type of Turtles is carried out the farthest in the genuine Testudinina. 

 it is the least so in 8phargis. 



SECTION V 



THE SKIX. 



The epidermis, the Malpighian layer, the corium, and the ossifications of the 

 latter, are to be found in all Turtles, but they show the greatest variety in 

 different families. We will analyze these different strata, proceeding from the 

 outside inwards. 



The Epidenim. The e2)idennis of the head is of great importance in charac- 

 terizing the ordex", the sub-orders, families, genera, species, and even the sexes of 

 Turtles. The practised observer may, from the sheath of the jaw alone, recognize 

 at least the genus. In all Turtles, the jaws are covered by a thick, epidermis, 

 which gives them the appearance of a genuine bill, more or less rounded in front, 

 witii sharp margins either smooth or denticulated. Such a bill is not found in 

 any other Reptile, nor in any order of Vertebrata, except in two Mammalia, in all 

 the Bii-d.s, and in the Tadpoles of the Batrachians. This horny sheath is errone- 

 oasly .said to be wanting in some Turtles. We find it in all, even in the Triony- 

 chidoe, where the jaws are covered by fieshy lips, but it varies greatly in thickness ; 

 while it is rather thin in the Emydoichc, it forms in the Cinosternoidaj a strong, 

 sharp hook, which is stronger still in Chelydra, and strongest in Sphargis, which is 

 veiy likely a carnivorous Turtle. In this last genus it has the form of a hook 

 bill, more powerful than even the Ijill of the South American Harpyia. 



On the toj) and on the sides of the head the epidermis forms either one continu- 

 ous layer, as in the Emydoidas, Cinosternoida), Chelydroida?, and Trioin-chida^ or it 

 is divided into a pavement of thicker plates, disposed either symmetrically, as in 

 Chclonia, or more irregularly, as in Testudo. On the under surface of the head, 

 on the chin and upper neck it is seldom thickened into distinct plate.s, but, no doubt 

 in order to provide for its greater movability, it is usually only divided by wrinkles 



33 



