Chap. I. THE SKIN. 2G1 



Till' C(>/(ir.s ill Turtles. The coloring of the lowest strata of the epidermis, the 

 so-called Malpighian layer, has not yet, so far as I know, been the object of a 

 special investigation. 1 deem it, therefore, worth our while to take up this point 

 more fully than other parts of the ordinal characters. The uppermost dry part of 

 the epidermis, the stratum conieum, which is so extensively developed in Turtles, 

 exhibits as usually by far the smallest part of the colors ; the most beautiful colors 

 being included chiell\' in tin- iMalpighian layer. That stratum, on the contrary, is 

 transparent, with a grayish lustre, like mica. Thus far only one Turtle is known 

 in which this dry, horny layer contains all the coloring matter, at least as far as 

 the colors are visible from outside, namely, Eretmochelys imbricata; and it is owing 

 to this extraordinary circumstance that in the dry plates of this Turtle (the tortoise- 

 shell) all their beautiful colors are preserved, even after the plates have been removed 

 from the Malpighian layer. A homogeneous brownish lustre may be seen with 

 the microscope in the epidermal cells, in all those j)laces of the plate where it 

 appears browni ; there is, however, no trace of pigment cells, nor of any fluid, and 

 that brownish color belongs only to the Avails of the cells.^ Still more intense 

 colors, often black, produced in the same way, are found in the thick plates of 

 nearly all land Turtles, for instance, in Testudo radiata, polyphemus, indica, etc., and 

 in some Chelonioida?, as in Chelonia Caouana and Mydas, but in all these the Mal- 

 pighian layer, lying beneath the plates, also takes part in forming the colors which 

 appear outside. 



The Malpighian layer, also called the pigment layer, is not only the matrix of 

 the epidermis, but at the same time the bearer of the pigment in Turtles. It is moist 

 and soft, and of very different thickness in different families, generally however thick 

 enough to be readily separated as one continuous membrane from the dry, horny 

 stratum which lies above it, as well as from the corium or bone which lies below. 

 It is composed of large, round, transparent cells, lying not in plane layers, but 

 rather imbricated. On, between, and beneath these cells lies the pigment, either 

 in cells or as a free fluid in lacunes, or in one continuous layer. Thus we have 

 to distinguish two diflerent forms, under which the pigment occurs in Turtles: 

 first, real pigment cells, which are always black or blackish bi-own, and filled with 

 browni.sh pigment molecules, upon the amount of which in a cell depends its more 

 or less dark tint; and secondly, a colored oily fluid, moistening generally the whole 

 Malpighian layer, and not contained in regular cells. Under this second form 

 appear the most various colons, such as the 3'ellow, red. bit)wn, and also sometimes 

 black tints of our different kinds of Turtles. The most diversified j)lay of colors 

 is produced by the condjinations of these free fluid colors, b}- their su{)erposition 



' As \vi' (iiid it al>o in soino phu'fs of the liuiiian btnly. Sec KoUikcr Gfwebclciiro. p. 98, § 43. 



