1907.] THE SCALES OF FISH. 755 



separate the dentine ridges, which may sometimes meet below. 

 Traces of the crenulations may still be seen. 



The underlying plate consists of an outer layer with very 

 large vascular spaces, and a lamellated inner layer. The skeletal 

 substance of which these are composed is quite similar to that of 

 the Fsammosteus plate. 



Thvis it appears that the shields of the Heterostraci, and also 

 the scales and dermal fin-rays, have all been evolved by the 

 combination of a covering of separate isolated denticles and an 

 underlying plate, and the theory of Williamson is confirmed in a 

 most remarkable manner by Traquair. But, and this is an 

 important point, the superficial tubercles and the plate should not 

 be compared to the pointed tip and expanded basal plate of an 

 ordinary Selachian placoid scale. Throughout these changes the 

 denticles i-emain essentially unaltered ; the inner and continuous 

 plate is evidently a quite distinct structure of separate origin — a 

 new skeletal support doubtless developed in the cutis, and with 

 which the denticles only came into secondary connection. In the 

 most specialised forms \Pteraspis) the latter almost entirely lose 

 their individuality, and acquire the deceptive appearace of being 

 merely the ornamental surface of a scale or plate ; yet they neither 

 lose their characteristic tooth-like histological structure, nor do 

 they really take part in the formation of the underlying plate. 



The true Scales. — Coming now to the fish with true scales, we 

 first of all study those scales which have hitherto been grouped 

 under the name "ganoid." Of these there are three types which 

 can be clearly distinguished. 



The Cosmoid Scale. — As an example of the first type, which may 

 be called the ^'^cosmoid " scale, we may take that of Megalichthys. 

 It has been so well described by Williamson (23), that little need 

 be said concerning its structure, which is shown in text-figs. 197 

 & 200. As usual, a basal layer is present composed of parallel bony 

 laminae of " isopedine," over which is a zone with large vascular 

 spaces. Near the surface these canals combine to a more regular 

 horizontal system forming a network passing round vertical canals, 

 which reach the outer surface of the scale. It is the openings of 

 these canals which give the pitted appearance to the shiny scales of 

 the " Orossopterygian " fish. Below the surface the canals expand 

 into conical chambers, between which pass upwards another set of 

 vertical canals ending above in pulp-cavities. From these radiate 

 a multitude of canaliculi. 



A section parallel to the surface and just below it shows the 

 conical chambers surrounded by a hexagonal pattern, formed by 

 the cut ends of the dentine tubules (text-fig. 1 97). A section further 

 down displays the pulp-cavities surrounded by the canals joining 

 the chambei's. This layer of vascular chambers and puljD-cavities 

 disposed with such beautiful regularity may be considered as the 

 highest development of " cosmine." Williamson gave this name 



