first generation 

 denticle 



'^^0;.r Jy^^^-^- 



unipolar ce 



enamel 

 J_ semidentine 



vascular pulp canal 



Figure 8-27. A, section of the armor of a Middle Devonian arthro- 

 dire, ro//icht/iys po/oris, which lacks the basal laminated component; 

 B, enlargement of section through a denticle to show the semidentine 

 and unipolar cell lacunae. (After Bystrow, \9S7\ 



cled part, a spongy middle zone, and a laminated base. There 

 are numerous canals for Sharpey's fibers. In forms without 

 enamel and with several generations of semidentine tuber- 

 cles, Sharpey's fibers penetrate the upper layers of the 

 armor as well. 



The ontogeny and phylogeny of the scales of arthrodires 

 has been described as follows. The earliest form had rela- 



tively thin armor consisting of a spongy bone, covered with 

 numerous denticles. The denticles had sharp tops and ridges 

 directed anteroposteriorly. It is quite possible that the pulp 

 cavities of these denticles were rather large. In the later forms 

 the armor was thicker but still without the basal laminar 

 portion, and the pulp cavities of the denticles had become 

 largely filled with prodentine osteons. 



In the next stage, the dentine of the denticles was re- 

 placed by semidentine and new layers of denticles were 

 formed over the old. In the late forms the laminar basal part 

 of the armor had been added, the denticles were reduced in 

 size, the amount of enamel covering them was decreased. 

 Several generations of tubercles were formed and over- 

 grown by bone. The spongy layer of bone underwent ex- 

 tensive reorganization and increased in thickness as the plate 

 was added to both to its outer and inner surface. In the 

 most advanced forms only the semidentine remnants of the 

 tubercles remained, associated with prodentine, and several 

 generations of these were deeply embedded in the bony outer 

 surface of the plate. The bony plate was now thick and had 

 a well-developed laminar base (isopedine). 

 Anfiorch The plates oi Bothriolepis (Figure 8-28) are quite 

 like those of the euarthrodire; the outer surface has knobs or 

 tubercles covered with a more or less laminar bone. Embed- 

 ded in this laminar bone are areas of semidentine suggesting 

 interred denticles. Below this is an outer spongy layer sep- 

 arated from an inner spongy layer by a thin zone of lami- 

 nar bone. The thick base of the plate is laminated. 



The structure of the antiarch plate exceeds the most ad- 

 vanced state of the euarthrodire. It appears that the armor 

 of the antiarch has been increased in thickness by the sep- 

 aration of the original spongy bone into an outer region of 

 small irregular channels and an inner region of large cham- 

 bers. These two regions are separated by a zone of laminar 

 bone which is indicated in at least one species of euarth- 

 rodire. 



unipolar cells 



lamellar 

 surface 

 bone 



spongy bone 

 amellor bone 



trabecular bone 



Figure 8-28. Stereosection of armor of Bot/iria/epis canadensis. 

 (After Goodrich, 1907 and Enlow and Brown, 1956) 



232 • THE SKIN AND ITS DERIVATIVES 



