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Into its real nature I do not propose to enter here, as I have 

 lately published a paper on the subject (Proceedings Royal Society, 

 February 1898, Abstract), but it will suffice to say that upon the 

 balance of its characters I regard it as enamel ; amongst other differ- 

 ences it has no collagen organic basis such as all known dentines and 

 bones possess. 



Its development is very peculiar, for unlike enamels, such organic 

 matrix as it possesses is furnished by a speciahsed layer of the den- 

 tine papilla, which corresponds in thickness to the ultimate so-called 

 enamel, and over this there lie enormous ameloblasts (70 to 80 [.i in 

 length) which doubtless take some share in the process of its calci- 

 fication. 



This layer and its resultant so-called enamel are fully homologous 

 with the similar structures in Lamna, only there, just as in Carchar- 

 odon figured by Dr. Rose, they overlie a mass of osteodentine (trabec- 

 ular dentine of Rose) instead of fine tubular dentine as in Carcharias. 

 By the application of his own criterion Dr. Rose is bound to call this 

 layer either enamel or true dentine, as it underlies the epithelial cells ; 

 as a matter of fact he calls it true dentine, in which I do agree with 

 him, as it differs very strongly in its chemical constitution, the 

 manner of its development, and in many other points from any known 

 dentine. 



Still, so far as the present purpose goes, it might have been 

 enamel and so does not necessarily invalidate his definition ; but the 

 case is quite different; when we come to the fine tubed dentine under- 

 neath it. 



The calcification of this tissue takes place not at the surface 

 of the dentine papilla, but commences along the inner 

 border of the specialised layer already alluded to as forming 

 the surface of the dentine papilla, and thence progresses inwards quite 

 in the ordinary way. Hence it is not underneath an epithelial layer; it 

 is separated from it by the whole thickness of the future so-called 

 enamel, and so by Rose's definition is not a true dentine. Moreover 

 it stands in precisely the same relation to the enamel organ as does 

 the osteodentine of his Carcharodon figure, or as does the osteodentine 

 (trabecular dentine Rose) of Lamna. 



But, as I have already mentioned, it is in every respect a most 

 typical true dentine and any criterion which would compel us to call it 

 anything else is ipso facto condemned. 



A new term has been introduced by Dr. Rose, namely Trabecular 

 dentine, which, as appHed by him, seems to me to result in grouping 



