APPENDIX. 791 



that in a Molluscan shell, for instance (e. g. Trigonid)^ we may have a 

 superficial membranous layer corresponding with Nasmyth's membrane, 

 a deeper prismatic layer, whose prisms precisely resemble those of 

 enamel on a large scale, and an internal laminated tubular layer, corre- 

 sponding with dentine ; and knowing, further, that these varieties of 

 structure thus arranged are (whatever view we take of shell-structure) 

 nothing but the result of the different modes in which calcified deposit 

 has successively taken place in the same organ, it is sufficiently obvious 

 that there are abundant analogical grounds for considering the enamel 

 and the dentine as modifications of one and the same tissue. 



Nor does the structure of the enamel in the Fish or the Batrachian 

 present any difficulty in the way of this view. It is, at most, indistinctly 

 fibrous and contains so large a quantity of calcareous matter in propor- 

 tion to the dentine, that the differences between the two may well be 

 supposed to arise as we believe they do from this circumstance 

 alone. 



In the higher Vertebrata, however, when the enamel in its young 

 state consists of definite fibres composed of organic substance, which 

 are added to the surface of the tooth only after the formation of a sub- 

 jacent scale of dentine, it becomes more difficult to comprehend the 

 development of the former. There appears to be three possibilities. 



1. What we call the primary scale of dentine is not, on the crown of 

 the tooth, dentine at all, but young enamel, becoming converted into 

 the latter structure, and not into the former, as development proceeds. 

 This appears, as first sight, a startling hypothesis enough ; but there 

 are, so far as we know, no means of disproving it. Young dentine can 

 only be known to be such by its relations ; in structure it is neither like 

 perfect dentine nor like perfect enamel ; but might readily be supposed 

 to be converted into either by variation in the quantity and mode of 

 deposition of its calcareous element. If this deposit be comparatively 

 small, leaving much of the organic basis, and not encroaching upon the 

 existing cavities, we have dentine ; increase the quantity of calcareous 

 salts, and break up the organic basis at the same time into fibres, and 

 enamel would be produced. 



2. The enamel is the indirect product of the prismatic cells of the 

 enamel organ, whose inner extremities pass into successive layers of 

 membrane, which are applied upon and indistinguishably unite with the 

 membrana preformativa over the whole surface of the developing enamel. 

 The laminated membrane thus formed receives a calcareous deposit, 

 and breaks up into the prisms of the enamel. 



This hypothesis likewise, at first sight, appears somewhat improbable, 

 but it may be strictly parallel with what occurs in the formation of 

 prismatic shell substance, where a laminated membranous substance is 



