pore 



— =_-enamel 3 to 5-micra thick 



mesodentine 

 mucous-sensory canal 

 ~ septum 

 vascular channel 



— ^ ' — —laminar bone 



basal chamber 



dentine 



first generation 

 denticle 



surface with pores 

 nail polygonal mucous canals 



large polygonal mucous canals, 

 radiating vascular canals 



irregular 

 vascular channel 



mesodentine 



Sharpey's fibers-' 



Figure 8-26. Armor and scale structure of osteostracans and thelodonts; A, section of armor of 

 Tremataspis mammiiiata; B, section of shield margin of Cepho/aspis sp?; C, stereodiogram of armor 

 structure of a cephalaspid; D, E, placoid scales of PMebo/epis a thelodont; F, semidentine scales of 

 Thehdus {upper Silurian), a thelodont. (A after Denison, 1947; B, D, E, F after 0rvig, 1951; C, after 

 Wangsjo, 1946) 



SCALES • 231 



