VERTEBRATA 



l'HYLUM YIII 



C 



\ 



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



roofing tiles. Cycloid scales 

 (Fig. 8, A-C) generally exhibit 

 a rounded or oval form and a 

 simple non - serrated hinder 

 l)order. In ctenoid scales 

 Fig. 8, D, E) there project 

 irom 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 

 rttructure 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 



small, round, concentrically marked calcareous bodies, which are most 



numerous beneath the primitive field, sometimes fused vvith each other into 



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 pärtly 



the head of many fishes {Arthrodira, 



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 



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 rieh sculpturing. 



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



Fig. S. 



Cycloid scales of A, Leiiciscns, B, Mormynts, and C, Nmicrates. 

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





Flu. 9. 



External surface of a scale of Pleuronectes, to sliow 

 tlie vacuities (x) between the raised concentric lines. 

 Very much magnified (after Baudelot). 



usually implanted loosely in the 



