Y PLACOID SKELETON 323 



enlarged spine-like scales in front of the dorsal fins, on the anterior 

 face of which it is well developed. The enamel is in turn covered 

 on its surface by an extremely thin membrane-like layer — the 

 enamel cuticle — and Huxley (1859) made out the important point 

 that this is continuous with the basement membrane of the epidermis 

 outside the limit of the scale rudiment. 



In the teeth of the higher animals, which as will be seen later 

 are simply modified placoid scales, the enamel is sharply marked off 

 from the dentine and it is usual to regard it as of totally different 

 origin namely as a kind of cuticular formation by the inner ends of 

 the enamel epithelial cells. The chief reasons for this view are the 

 sharp differences in appearance and composition from the dentine in 

 these higher Vertebrates, and the fact that the cells of the enamel 

 epithelium undergo a shortening as the enamel layer thickens — as if 

 the inner ends of the epithelial cells were undergoing conversion 

 into enamel from within outwards. 



It is however Curiously difficult to find evidence sufficiently con- 

 vincing to justify the almost universal acceptance of this idea even 

 as regards the higher Vertebrates. And in the case of the Fishes the 

 evidence — such as the location below the basement membrane and 

 the frequently quite gradual transition between the so-called enamel 

 and the dentine— strongly supports the idea that the former is simply 

 a modification of the outer layer of the dentine. 



The basal edge of the cone of dentine comes to spread outwards 

 all round parallel to the surface of the skin as irregular trabeculae 

 forming a strong basal plate by which the scale is firmly fixed 

 in the dermis. This basal plate is usually of homogeneous appear- 

 ance but its substance shows a gradual transition to the typical 

 dentine of the spine, and in the case of Callorhynchus (Schauinsland, 

 1903) the basal plate as a whole shows, just as it does in the ancient 

 fossil Coelolepids, dentinal structure. There seems then no reason 

 to doubt that the basal plate is in its nature closely allied to dentine 

 or in other words that it is bone in the broad sense of the term. 



Teeth. — A section across the jaw of an ordinary Dog-fish is 

 sufficient to demonstrate the important morphological fact of the 

 homology of the teeth and the placoid elements of the skin. Teeth 

 are simply placoid elements belonging to that portion of the outer 

 skin which is carried inwards to form the stomodaeum. Or con- 

 versely the spines of the placoid scales are simply teeth which have 

 not been carried inwards into the stomodaeum. In accordance with 

 this the placoid scales were long ago (1849) named, by Williamson, 

 dermal teeth. The demonstration of the homology in detail will 

 be found in a classical paper by 0. Hertwig (1874). 



The lining of the buccal cavity being morphologically part of 

 the outer skin the probability is that originally teeth or placoid 

 elements were distributed equally all over it. But in the evolution 

 of the Vertebrata there has clearly taken place a restriction of the 

 teeth to particular parts of the lining where they can be most 



