PISCES 



teeth. Placoid scales often 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 regular rows, and 

 are movably articulated by a 

 tooth-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 fig. i. 



(ganoine) ; it is usually thick Scale of Cosmoptychius stHatits, Ag. 

 n 1 • • , • 1 1 sp. Outer (A) and inner (B) aspects. 



and shinnig, sometimes dark- Lower Carboniferous. 3/^. 



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 layers include numerous lacunae 



Fio. 5. 



Two scales of Poly- 

 pterus bichir, Bonap. 

 Kecent. Inner aspect. 

 Nat. size. 



Fig. 6. 



Vertical section of a scale of Lepidosteus. 

 Recent. .5, Enamel ; d, Dentine tubules : 

 h, Haversian canal. 300/-^ (after O. Hertwig). 



Fig. 7. 



Vertical section of a scale of GlyptoUpis, 

 highly magnified. Devonian, a, h, Layer 

 of vasodentine with spindle-shaped bone 

 lacunae ; c, Bony layer with Haversian 

 canals ; d, Layer with tine, branching 

 dentine tubules (cosmine) ; e, Enamel (after 

 Pander). 



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

 traversed only by fine dentine tubules. 



3. Cycloid 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 



