mTEGU]\IENT OF VERTEBEATA. 



423 



skeleton. lu many cases we know but little as to the development 

 of these structures, but they may all be reckoned among osseous 



Fig. 218. Vertical section through the skin of an Embryonic Shark. C Corinni. 



c, c, c Layers of the corium. d Uppermost layer. 2^ Papilla. E Epidermis, e Its 



layer of columnar cells. Enamel layer. 



formations, to which indeed they completely correspond in the 

 higher divisions. 



The dermal denticles (placoid scales), which are distri- 

 buted over the whole of the in- 

 tegument in the Selachii, may be 

 regarded as the structures from 

 which the various forms have 

 been derived. In them we distin- 

 guish a basis, which is inserted into 

 the corium, and is ordinarily rhom- 

 boidal in shape, and a portion, which 

 stands out from it, and which ordinarily 

 has its apex directed obliquely. This 

 is covered over by the epidermis. In 

 some parts, as, for example, on the head, 

 they often have a bombous surface, 

 and are set irregularly ; while on the 

 trunk they are generally set in per- 

 fectly regular and obliquely running- 

 rows (Fig. 219). They are developed 

 on papillas of the corium (Fig.218, 

 p), which are covered over by a 

 layer developed from the epider- 

 mis : this secretes an enamel-like 

 substance on the projecting por- 

 tion of the papilla, while the body 

 of the papilla is ossified from the 

 tip downwards. The epidermis and corium, therefore, both share 

 in the formation of these structures. There is a central cavity in the 



Fig. 219. Dermal denticles of 



Ceutrophorus calceus (a little 



magnified). 



