VERTEBRATA 



PHYLUM VIII 



B 





developed in special folds of the cutis, and generally overlap one another like 



roofing tiles. Cycloid scales 

 (Fig. 8, A-G) generally exhibit 

 a rounded or oval form and a 

 simple non - serrated hinder 

 border. In ctenoid scales 

 (Fig. 8, D, -E) there project 

 from the hinder border small 

 points and denticles, which are 

 sometimes arranged in several 

 series one behind the other, 

 and may indeed cover a con- 

 siderable part of the hinder 

 surface and have the same 

 structure as the superficial 

 layer. From the so - called 

 primitive field divergent lines 

 radiate chiefly forwards and 

 backwards, and sometimes also 

 pass into reticulations ; these 

 lines being merely fissure-like 

 interruptions in the outer layer 

 (Fig. 9). Between the upper 

 parallel layers of the fibrous 

 connective tissue of the base 

 there are often intercalated 

 calcareous bodies, which are most 

 fused with each other into 



Fir;. S. 



Cycloid scales of A, Leiiciscus, B, Mormyrus, and C, Naucrates 

 Ctenoid scales of D, Solea, and E, Holacanthus. Much magnified. 



small, 



round, concentrically marked 

 numerous beneath the primitive field, sometimes 

 a continuous layer, and here and there 

 exhibit bone cells. Such scales are not 

 essentially different from very thin 

 ganoid scales. 



The large plates, shields, spines, etc., 

 which partly cover the body and partly 

 the head of many fishes (Artlirodira, 

 Siluridae, Acipenseridae), consist either 

 of uniform bone substance, or, like the 

 ganoid scales, of layers of ostaine, 

 dentine, and enamel. 



Large spines (Ichthyodorulites), 

 (Fig. 10) generally occur on the front 

 margin of the median, and more rarely 

 of paired fins, or on the head of carti- 

 laginous fishes (Selachii). They are usually implanted loosely in the 

 muscles by an elongated base, only exceptionally (Chimaera) articulated with 

 a lower piece. Like the scales and teeth of Placoid fishes they consist 

 either entirely of dentine or of dentine and vasodentine. Ichthyodorulites 

 often attain a remarkable size, and occur isolated in a fossil state, while the 

 Palaeozoic examples are sometimes characterised by a rich sculpturing. 



The outer projecting parts of the fins are also to be regarded as dermal 



Fig. 9, 



External surface of a scale of Pleuronectes, to show 

 the vacuities (as) between the raised concentric lines. 

 Very much magnified (after Baudelot). 



