Chap. I. THE SKIN. 263 



The Conitm. A thorough analysis of the corium is of the greatest interest 

 in the study of the Turtles, because this part of the skin is the scat of all those 

 deposits of lime which compose their dermal skeleton. The corium is composed of 

 two very different layers: first, a layer of elastic fibres, immediately imder the stra- 

 tum Malpighii, consisting of the same kind of anastomozing, or ratlier net-like, elastic 

 fibres tliat we find in the walls of the arteries, etc. ; secondly, a layer of a tissue 

 consisting of smooth, long fibres crossing each other, and interwoven sometimes 

 more regularly, as in the Trionychidtv, or irregularly, as in Sphargis. According to 

 the numerous sections which we have made, a dej^osition of lime generally takes place 

 only in the elastic fibres, while the fibrous tissue lying beneath is resorbed. At least 

 we find in all ossifications, when young, the arrangement of elastic fibres still very 

 distinct; and Sphargis, in which a bony shield of about two lines in thickness 

 begins immediately under the Malpighian layer, seems to show this particularly well. 

 Under this follows a thick, coarse, fibrous tissue, in which there are no ossifications 

 at all ; under this, finally, follows the skeleton. In sections made in different direc- 

 tions through the shield, Ave see clearly the character of the ossifications, as well as 

 that of the skin which does not ossify, and that of the skeleton proper, which in 

 most Turtles is very much affected by the ossification of the skin. A section 

 through the soft but thick margin of the dorsal shield of Trionyx ferox, in which 

 no ossifications take place, shows first a thin epidermis, then a thicker layer of 

 elastic fibres, then many layers of fibres crossmg each other regularly and producing 

 by the regularity of their knees those seeming layers of the skin which are so strik- 

 ing to the naked eye in any transverse section. Another section through a dermal 

 ossification of the sternum of the same Turtle, shows the difference between the 

 true skeleton bone, with its very regular structure, bone-holes, etc., and the deraial 

 bone above it, in wliich many canals run through, piercing it in different directions, 

 and in which the bone-holes also are more irregularly disposed, show-ing its origin 

 from elastic fibres. This is still more evident in a section through a younger 

 ossification in Chelonia Mydas, where the roundish or longitudinal holes of the 

 elastic fibres are very distinct. Again, another section near the former, where the 

 ossification has not yet begun, shows the character of the elastic tissue when it 

 is about to be ossified. A horizontal section through the bony shield of Sphargis, 

 which, as stated above, nowhere touches the bone, is also very characteristic. This 

 structure furnishes of itself sufficient evidence of tlie incorrectness of the views 

 which Cuvier' and others entertained, that the w'hole lK)ny shield of Turtles is pro- 



' AVillidut inakiiig any distinction ln'twoen llic also Gfoffroy, (Mom. dii Museum, vol. xiv.,) consider 



dermal and llic true skeleton, Cuvicr (Le9ons d'Ana- tiic carapace as formed entirely liy the dilalalion of 



tomie comparee. Sd edit., vol. i., p. 2G3, and Osse- the verlelira> and the ribs. Cams (Urtheile, etc., p. 



ments fossiles, vol. v., 2d part, p. 195), and with him 150; was the first to show that a considerable portion 



