Acanthodian 



The small scales of the acanthodian show a wide range of 

 structure (Figure 8-29). In Nostolepis (Upper Silurian) the 

 scale begins as a small mesodentine tooth without an enamel 

 cover. At this stage it qualifies as a placoid scale. Growth 

 involves the addition of material completely enclosing the 

 old. In this way successive layers of mesodentine were added 

 to the crown. These are thin above and thick at the mar- 

 gins; new layers are also added to the bony base. Other 

 scales, resembling those oi Nostolepis, have mesodentine tuber- 

 cles set on a bone base. Second generation denticles may 

 overlie the original denticles. 



In Acanthodes the scale is formed of numerous shells of 

 dentine applied successively over an original denticle, and 

 the whole is attached to a laminated, acellular bony base. 

 The vascular supply for the dentine comes in at the neck of 



initial denticle 



growth layers 



luiar bony base 



vascular pulp canal 



two generations of denticles 



C5^ 



bony base with enclosed cells 



laminated ganoin 



dentine 



^neck canal 



bony base without enclosed cells 



Figure 8-29. A and B, sections of scales of Nostolepis from the Upper 

 Silurian (after 0rvig, 1957). C, section of scale of Aconthodes, Penn- 

 sylvanian, (after Goodrich, 1 907). 



the scale; the base of the scale is penetrated by radiating 

 Williamson's and Sharpey's canals. The former were oc- 

 cupied by processes of the osteoblasts of the bony base. 



The acellular material of the Acanthodes scale contrasts 

 with the cellular nature of the Nostolepis type. In terms of 

 their scales the acanthodians are thus of two types. 



Chondrichthyes 



The placoid scale of the shark has already been described 

 (Figure 8-22). A point of special interest is the nature of the 

 enamel. This has been called "fibrodentine" since it is 

 formed by calcification of the fibrous material formed be- 

 tween the epidermal enamel organ and the dentine (Figure 

 8-,30). This material occasionally encloses odontoblasts and 

 is penetrated by the terminal tips of the dentinal tubules. 



Among sharks there is a great variation in scale structure. 

 By way of contrast, the scale of a ray, Dasybatus, is broad 

 and flat with an acellular bony base (Figure 8-31). The 

 dentinal layer is supplied by branching vascular canals 

 which penetrate the base and the neck of the scale. The 

 bony base shows concentric laminae and is perforated by 

 canals of Williamson. These vascular canals also contain the 

 bone cells of the base. Sharpey's fibers penetrate the base 

 and anchor the bone to the fibrous dermis. Cellular bone is 

 lacking in living chondrichthyes but was present perhaps in 

 early fossils (edestids of Permian). 



oniceilulor glond stratum germinolivum enomel 



germinal layer of epidermis 



Figure 8-30. Development of placoid scale of the shark. A, early stage 

 in Squa\us (after Klootsch, \ 894); B, later stage in Hepianchus (after 

 O. Hertwig, 1 874). 



SCALES 



233 



