SCALES 



217 



According to Williamson's original theory the scales and plates 

 of the Osteichthyes have been formed by " the confluent aggrega- 

 tion and superficial depression of a number of placoid teeth " over- 

 lying and becoming fused Avith a lower bony plate. We have seen 

 how admirably this theory accounts for the structure of the 

 exoskeleton of the Pteraspidomorphi (p. 195); will it equally well 

 .account for that of the ganoid scale 1 



We must carefully distinguish between two very different kinds of 

 scales hitherto called ' ganoid ' (Goodrich [1 78]). The first, which is 

 the true ganoid scale, occurs in the Actinopterygii. The second may 



ff- 



FIG. 190. 



Scale of Megaliehthys llibberti, A. ; Carboniferous, England. A, piece of a thick transverse 

 section, much enlarged. B, section through the hind edge, enlarged. C, outer view of a scale. 

 ac, anterior region covered by next scale ; c, large vascular cavity ; ch, chamber of cosmine 

 layer ; dt, canaliculi of cosniiue ; g, thin outermost shiny layer ; h, irregular vascular canals ; 

 ;, bony inner layer or isopedine ; o, opening of chamber on surface ; p.c, pulp-cavity from 

 which canaliculi radiate ; i-.c, vertical canal. 



be called the cosmoid scale, and is found only in the Dipnoi and Osteo- 

 lepidoti (Figs. 189, 190). The latter is formed of three layers: a 

 middle layer of bone with large vascular anastomosing channels ; an 

 inner layer of bony "isopedin," in which the matrix is deposited in 

 successive laminae, and the cells are arranged in parallel planes ; and 

 an outer layer of " cosmin " (Williamson [496]). This cosmine is of 

 complex structure, being formed of cell-less matrix in which vertical 

 vascular spaces alternate with ' pulp-cavities ' from which radiate 

 fine canaliculi. It resembles dentine (Fig. 1 90). A thin layer of 

 enamel or vitrodentine covers its outer surface. 



The ' ganoid ' scale, on the other hand, consists of concentric 

 layers of skeletal substance, passing both above and below. Two 



