THE SCALES OF FISH. 



75S 



1907.] 



surrounding irregular spaces. Since scleroblastic cells are free to 

 pass on both sides of the ingrowing base, it can increase in size in 

 ail directions. 



One may summarise the chief points thus : — The placoid scale, 

 or denticle, begins as a cone of dentine deposited by mesoblastic 

 scleroblastic cells below the epidermis, in continuity with the 

 basement membrane ; a basal plate may be present in the form of 

 a direct extension inwards of the cone, never as a separate 

 element which becomes fused on to it secondarily ; both the cone 

 and the plate are composed of dentine or some allied substance, 

 never of true bone ; the cone may pierce the epidermis, when fully 

 grown. 



(From Lankester's ' Treatise on Zoology,' by permission of Messrs. A. & C. Black.) 



Diagrams of the structure and development of the dermal skeleton of A, an early 

 stage, and B, later stages of Elasmo'branchs ; C, Thelodus ; D, Fsammosteus ; 

 E, Pteraspis, all in section at right angles to the surface ; the dentine is black. 

 F-I. Enlarged views of the outer surface of the dermal skeleton of F, Thelodus 

 head, G, Thelodus i;ail, H, Fsammosteus shield, and I, Fteraspis shield, hf., 

 expanded basal plate; bm., basement membrane; ct., connective tissue; dc, 

 dentine cap; ep., epidermis; I., bony lamellie ; p., pulp-cavity; r., smiace 

 ridge ; tr., bony trabeculse of vascular layer. 



Such denticles are found in the Selachii and Holocephali, which 

 are devoid of other forms of scaling. Probably they also occur m 

 the Pleuropterygii (Oladoselachii) and Ichthyotomi (Pleuracan- 

 thodii) ; though details concerning the histological structure of the 

 scales of these fish are still lacking. Rarely the denticles seem to 

 fuse together, as for instance in Hyhodus (28). The circumorbital 



