HIGH SCHOOL -ZOOLOGY. 101 



Tlie most remai-kable point in regard to the skin is the 

 development in it of certain bony plates, which, however, are 

 so closely related to the internal skeleton, as to be more properly 

 dealt with in connection therewith. In other respects we have 

 to notice here as well as in all the other Sauropsida the great 

 development of the horny layer of the epidermis, no longer 

 confined to the extent of one or two layers of cells, or locally 

 thickened here and there, but developed into the chai-acteristic 

 clothing of scales, scutes, shields or feathers. In the Snapper 

 we distinguish two kinds of these epidermal appendages, the 

 regular shields which cover the dorsal and venti-al surfaces 

 of the trunk, and the smaller and less regular scales and tuber- 

 cles of the rest of the surface. In addition to these are the 

 formidable claws with which the distal or ungual phalanges of 

 the digits are provided. Although all of these structures are 

 formed of horny epidermal cells, yet the underlying mucous 

 cells replace them from below as they are worn off above, and 

 the corium likewise partakes in their formation. The dorsal 

 sm-face has three rows of larger cariuated shields (five unpaired 

 vertebral, and four paired costal) surrounded by twenty-five 

 smaller ones, of which eleven at each side are called marginal, 

 while that in front is nuchal, and the two behind caudal. These 

 shields ai-e .separated by very scanty connective-tissue from the 

 underlying bones of the exoskeleton. Similarly, on tlie ventral 

 surface thex'e are six paired shields named from befoi'e back- 

 ward, gulai', postgular, pectoral, abdominal, preanal and anal, 

 of which the abdominal are the largest. It is these large epi- 

 dermal shields, which in one of the marine turtles, furnish the 

 tortoise-shell of commerce. (Fig. 74). As in the other mem- 

 bers of the order, the jaws are provided with horny sheaths 

 like a parrot's beak instead of teeth, aiad the terminal hooks of 

 these are of considerable size in the Snapper. 



After the e]>i(lermal shields have been removed, it is seen 

 that their outlines do not correspond to those of the bony 



