denticle 



three generations of denticles 

 enamel 



mesodentine (no enamel) 



denticle 



mucous-sensory 

 pore 



enamel 



aspidin with Sharpe/'s f 



Figure 8-25. Armor and scale structure of heterostracans. A, stereodiagram of piece of plate of 

 Eryptychius sp? from the Upper Ordovician; B, frypfychjus sp?, section of scale with three genera- 

 tions of denticles; C, Pycnaspis sptendens, section of plate; D, section of armor of Sanidaspis siberica 

 cut along line d shown in E; E, surface pattern of enameled ridges in Sanidaspis; F, horizontal section 

 of armor in Sanic^aspis cut along line f shown in D; G, section of armor of Drepanospis sp?. (A to C 

 offer 0rvig, 1958 and 1959 (Ms); D to G after Bystrow, 1959) 



basal layer and filling in of the cavernous interior. The fully 

 developed armor fused viiith underlying endoskeletal elements. 



Arthrodires 



Euarthrodire The armor of some species of arthrodires has 

 been described, and, in some of these, enamel-covered den- 

 ticles are exposed on the surface (Figure 8-27). These 

 denticles are partly embedded in bone, and, in some species. 



several layers of denticles are embedded, one above the 

 other. The embedded denticles are usually without enamel 

 but some may retain traces of it. 



The denticles have restricted, vertical pulp cavities en- 

 closed by thin osteons of bone. This bone is peculiar in having 

 unipolar osteoblasts and as such it might be described as 

 prodentine. The semidentine is restricted to a narrow zone 

 near the surface of the denticle, just below the enamel. 



The armor has three layers of material; the outer tuber- 



230 • THE SKIN AND ITS DERIVATIVES 



