enamel 



vascular pulp canal 



dentine 



Sharpey's fibers 



osGl canal 



acellular lamellar bony base 

 Figure 8-31 . Section through the scale of a ray, Dosybotus sp?. (After 

 0rvig, 1951) 



In the holocephalans, denticles occur only on the claspers, 

 and in the young there may be a double row of small den- 

 ticles along the back. The denticles and spine structure are 

 like those of the shark. 



General observations The placoid scale of the shark has 

 been described as a basic skeletal unit, and it is apparent 

 that denticles of one shape or another are characteristic of 

 the bone and scales of vertebrates. In very primitive sharks, 

 small units, called lepidomoria, are found (Figure 8-32). In 

 more advanced sharks, the simple tooth-like lepidomoria 

 tend to fuse together to form larger compound units. It has 

 been assumed that the history of vertebrate scales and plates 

 has been one of marginal aggregation of these lepidomorial 

 units around a central element — this growth was accom- 

 panied by acquisition and thickening of the base. The shark 

 placoid scale is peculiar in its irregular shape as contrasted 

 with the regular-shaped scales of other groups. 



Actinopterygian fishes 



Ganoid scale The scales and the dermal bone of primitive 

 actinopterygian fishes are described as ganoid because of 

 the "shiny," enamel-like material covering their surface. As 



already pointed out, the fact that ganoin may be produced 

 by the dermis, and not by the overlying epidermis does not 

 mean that, in essence, it is not phylogenetically the same 

 material. If the material occurs on the tooth of the fish, it is 

 described as enamel; if it occurs on the scale, it is described 

 as ganoin. This distinction does not seem quite proper. 



In the primitive ganoid scale, the layers of enamel overlie 

 a zone of dentine. The dentine has a network of vascular 

 canals below it; these open at the neck of the scale as well 

 as through the base and the outer surface by one or a few 

 channels. The base of the scale is made of laminae of bone 

 (isopedine) which continue up around the margins of the 

 scale to meet the layers of "ganoin." 



One of the earliest paleoniscoid fishes, Cheirolepis, has a 

 small scale with only a single basal vascular canal and a 

 posterior neck opening (Figure 8-33). In other, later paleo- 

 niscoid fishes the vascular system is somewhat more complex, 

 opening both dorsally through the ganoin and ventrally 

 through the bony plate. 



The manner of growth of this type of scale is indicated 

 by its structure. It starts with a small central unit (lepido- 

 morium) and grows marginally by distinct increments, each 

 of which is accompanied by a fresh deposition of enamel over 

 the outer surface of the scale and the addition of a basal 

 lamina of bone. In some paleoniscoids, the early denticles 

 may be covered by second-generation denticles; zones of 

 dentine may lie between layers of enamel. With concentric 

 growth, new vertical vascular channels may be enclosed in 

 the hard substance. These Williamson's canals extend ver- 

 tically through the base to the pulp cavities of the dentine. 



In the paleoniscoids, two types of scale development are 

 observed: one retains the vertical vascular channels, the 

 basal canals, of the component lepidomoria; the other has 

 only one or a few large vertical vascular channels (Figure 

 8-34). In the latter the pulp canals are served by neck and 

 dorsal vascular channels. The basal canals are called Wil- 

 liamson's canals. These were thus primarily vascular chan- 

 nels (Figure 8-35), but, as the scale evolved, contact with the 

 vascular plexus of the dentine was lost. The canals now be- 

 came narrower and served only as areas for the retreat of 

 the osteoblasts from the osteons of the basal part. 



Scales without Williamson's canals are the more common 



..vascular canal 

 ' ( \ bone 



nitial lepicJomorium 



dentinal crowns 



bony base with enclosed cells 



Figure 8-32. Scale structure in primitive sharks. A, a leprdomorium, B, section through scale of an 

 edestid shark from the Permian of Greenland; C, horizontal section through crown of scale in B. (After 

 0rvig, 1951) 



234 • THE SKIN AND ITS DERIVATIVES 



