1907.] THE SCALES OF FISH. 763 



similar small denticles fused to the surface of the scales of 

 Lejndosteus. 0. Hertwig, in a series of valuable papers on the 

 dermal covering of fish, contributed a number of interesting 

 observations on its structure and develoj)ment. But, neglecting 

 the work of Williamson, the importance of whose researches he 

 failed to appreciate, Hertwig does not appear to have understood 

 the growth of the ganoid scale, and moreover revived the old and 

 fundamental error of calling the outer layer true enamel. Tracing 

 the origin of all the scales and dermal bones of Lepidosteus 

 to the fusion of small plates bearing a denticle, homologous 

 with the placoid scale of the Elasmobranch, Hertwig concluded 

 that originally these fish were provided with a general covering 

 of denticles, that by concrescence their basal plates gave rise to 

 scales, that enamel was deposited where such plates reached 

 the surface, and that subsequently the denticles tended to 

 disappear. Klaatsch, in his important paper on fish-scales (10), 

 supports Hertwig's general theory ; but considers that each scale 

 of the higher fish represents a single denticle of which the basal 

 plate has become much enlarged. " Die Ganoidschuppe ist der 

 Placoidschuppe homolog, in so fern sie die alte Einheit fortfiihrt. 

 Sie entspricht in der Hauptmasse der Basalplatte, und zwar den 

 tieferen Theilen derselben. Der Spitzentheil der Placoidschuppe 

 ist rudimentar geworden. Die Ganoinschicht der Lepidosteus- 

 schuppe entspricht den oberfljichlichen Theil der Basalplatte. 

 Den Ziihnchen der Lepidosteusschuppe kommt keine morpholog- 

 ische Bedeutung zu." (p. 155.) 



Nickerson also compares the ganoid scale to the placoid denticle 

 on the supposition that the basal plate of the latter has given rise 

 to the scale. The separate development of the dentine cone and 

 of the plate in Lepidosteus, he would interpret as due to secondary 

 modification (13). 



The denticles on the scales of Lepidosteus and Polyptems are 

 generally spoken of as degenerate vestigial structures usually 

 absent in the adult, except in certain restricted regions. But 

 there is reason to believe that the denticles are much more 

 abundant and regularly distributed than is commonly supposed. 

 They are to be found in adult specimens of both genera on the 

 scales of the trunk where these have been protected from rough 

 usage. Their absence in many specimens is due to a great extent, 

 if not entirely, to their having been rubbed off". As Nickerson 

 (13) has well shown in Lepidosteus, the denticles develop just 

 like those of Elasmobranchs, on the surface of the mesoblastic 

 tissue, and afterwards become fused on to the scale, in the 

 formation of which they take no real part, except in so far as they 

 may occasionally get buried in it. The same is the case in 

 Polypterus, as my observations show. 



The comparison of the denticle and ganoid scale with the conical 

 apex and basal plate of the placoid denticle seems, therefore, to 

 be fundamentally wrong. As already insisted upon, the basal 

 plate is merely an extension of the denticle cone ; there is no 



Proc. Zool. See— 1907, No. LII. 62 



