PISCES 



teeth. Placoid scales ofteii fall out and are replaced like teeth by successional 

 tubercles. 



2. Ganoid scales (Figs. 4-7) attain a larger size than placoid, and usually 

 Cover the whole trunk. They are of rhombic or rounded shape, and in the 

 latter case overlap one another 



like roofing tiles. Those of ^ ^' 



rhombic form are usually 

 arranged in regulär rows, and 

 are movably articulated by a 

 tootli-like process of the upper 

 margin which fits into a corre- 

 sponding hollow on the inner 

 side of the adjoining scale. 

 The exposed outer face is 

 covered by an enamel layer 

 (ganoine) ; it is usually thick 

 and shining, sometimes dark- 

 coloured, smooth, rugose, or 

 ridged, and beneath it there is a basal plate consisting of bone substance and 

 pierced by numerous canals. Among the Palaeozoic genera, indeed, the basal 

 plate shows great complexity. The deeper lavers include numerous lacunae 



'^^ 



!^ Scale of Cosmoptychins sfrlutna, Aji'. 

 sp. Outer (A) and inner (B) aspects. 

 Lower Carboniferons. '■^/i. 



Two scales of Poly- 

 pterus bichir, Bonap, 

 Recent. Inner aspect. 

 Nat. size. 



Fio. G. 



Vertical section of a scale of Leptdosteiix 

 Kecent. .S', Enamel; d, Dentine tubules 

 h, Haversian canal. »oo/^ (after O, Hertwig). 



Vir.. 7. 



Vertical section of a scale of Gli/ptolepis, 

 highly niagnified. Devonian. «, b, I^yer 

 of vasodentine with spindle-shaped bone 

 lacunae ; c, Bony layer with Haversian 

 canals ; d, Layer with tine, branching 

 dentine tubules (cosniine) ; e, Enamel (after 

 Pander). 



(bone corpuscles) and Haversian canals, while the upper layers as a rule are 

 traversed only by fine dentine tubules. 



3. Cydoid and ctenoid scales are exactly alike in general characters, being 

 thin, elastic, and of rounded, elliptical, four-, five-, or six-sided shape. They 

 consist of a homogeneous, transparent shining surface-layer of phosphate of 

 lime, and of a base of connective tissue deficient in alkalies. The scales are 



