1907.] 



THE SCALES OF FISH. 



767 



on tlie top of the scale (PL XLIY. fig. 1 2). Whether these tubercles 

 represent modified denticles remains to be proved. A careful 

 study of better preserved material, and of other genera, might 

 settle this point. 



Turning now to the Dipnoi, we find that Dipterus has typical 

 cosmoid scales (p. 765). All the living genera have scales of very 

 uniform structure (Wiedersheim 25, Giinther 5, Klaatsch 10); con- 

 sisting of a basal layer of isopedine lamellae, little if at all calcified, 

 and an outer calcified layer. The latter closely resembles the outer 

 vascular and ornamental layers of the scale of the Rhizodontidse^ 

 A horizontal network of canals and space runs through the base 

 of the calcified region, and opens by numerous pores between 

 more or less well-defined ridges. A m.ultitude of very small 

 spines projects from the whole surface of the scale. The ridges 

 and partitions separating the vascular spaces grow by the additioix 



Text-fis. 201. 



vc. 



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



Enlarged view of a small portion of the scale of Flianero^pleuron curtum Wht. ; 

 Upper Devonian, Canada, i., inner bony layer ; sp., surface spinelet ; vc, vascular 

 space. 



of new laminae of bony substance without cells ; and the spines- 

 are merely processes of the calcified layer developed in just the 

 same manner. They are obviously analogous to the tubercles on 

 the scale of the Rhizodonts, and like them are found all over the 

 posterior covered as well as generally over the anterior exposed 

 region. These spines have nothing to do with denticles, with 

 which they have been compared by Wiedersheim (25) ; and 

 differ from them radically both in structure and development. 

 Wiedersheim's theory has already been disposed of by Klaatsch 

 (10). Now it is very interesting to find that this peculiar 

 " Dipnoan " type of scale is already perfectly differentiated in the 



