1 88 



ACANTHODII 



the body. The paired fins are of similar form, with a strong anterior 

 spine and thin membranous web, supported, however, in some cases 

 by ceratotrichia with calcified base (Reis [352]). The mouth is 

 wide, the orbit far forward, and the nostrils were probably placed 

 as in Selachians. The five gill-slits opened independently to the 

 exterior. 



The whole body is covered with minute closely fitting dermal 

 scales (Fig. 156). These scales, which are generally loosely com- 

 pared to placoid scales, differ fundamentally from the denticles of 

 Elasmobranchs (Reis [352], Goodrich [178]). They have a deep- 

 lying solid base expanding superficially into a more or less quad- 

 rangular shiny plate sometimes ornamented, or produced to a point. 

 There is no pulp-cavity ; the scale is made up of concentric layers 



.9 



-dt 



FIG. 157. 



Transverse section of the scale of AcantJiades sp. ; Lower Carboniferous, Edinburgh. 

 dt, branching canaliciili ; g, outer shiny layer ; I, inner more opaque layer. 



of bony substance without bone-cells ; minute branching canaliculi 

 penetrate into this substance from the outside, and it therefore 

 has some resemblance to dentine (Fig. 157). The outer layers 

 resemble ganoine, and there is no enamel. In fact, the Acanthodiau 

 scale, except for the absence of cells and vascular channels, is just 

 like a small ganoid scale (especially like the scale of Cheirolepis, 

 p. 311). It differs so markedly from the denticle that it is 

 impossible without intermediate forms to say whether it has been 

 derived from the Elasmobranch placoid scale. On the other hand, 

 the resemblance it bears to the ganoid scale is very possibly clue to 

 convergence. 



The scales spread on to the caudal fin, in rows simulating 

 lepidotrichia, and to a less extent on to the other fins also. On 

 the head they increase in size and form a covering of close-fitting 

 plates. Four or more similar plates surround the orbit. The 



