218 



OSTEICHTHYES 



distinct types of true ganoid scales can be distinguished [178], The 

 first the Palaeoniscoid type has four layers : isopedine below, a 

 vascular layer, a layer with canaliculi branching from a horizontal 

 system of canals (comparable to the cosmine layer), and lastly, a 

 layer of successive lamellae of enamel-like, cell-less substance the 

 ganoine (Fig. 191). The second the Lepidosteoid type has the 

 ganoine above and the isopedine below, but only a few irregular 

 transverse vascular channels; there is no real cosmine-like layer, 

 and the scale is characterised by a number of small tubules passing 



FIG. 191. 



Eurynotus crenatus, Ag. ; Lower Carboniferous. A, diagrammatic and much enlarged view of 

 a piece of the scale. 13, enlarged outer view of a scale. C, transverse section of a scale, en- 

 larged, a, anterior covered region ; a.p, articulating process ; c, tine canaliculi of cosmine 

 layer ; g, ganoine layer ; h, system of horizontal canals ; i, isopedine layer ; o, opening on 

 outer surface of vertical canals ; p, posterior exposed shiny surface ; s, outer surface ; v.c, 

 vertical canal. 



inwards at right angles to the lines of growth from the lower and 

 lateral surfaces (Fig. 192) (Reissner [355], Scupin [394]). 



Whilst the cosmoid scale grows only by the addition of new 

 cosmine round its edge, and of new layers of isopedine on its under 

 surface, the ganoid scale grows by the addition of concentric layers 

 over its entire surface, each of which contains elements of all the 

 strata of the scale. No living fish has a cosmoid scale. It is 

 probable that the ganoid is derived from the cosmoid type of scale, 

 which may be supposed to have sunk away from the epidermis, 

 becoming inclosed in a mesodermal pocket by which the new com- 

 plete concentric layers are deposited. If, as Williamson suggested, 

 the cosmine or dentine layer represents an original set of super- 



