66 Royal Society. 



than his predecessors, entering into the question of the development 

 of the tooth both generally and histologically (though apparently in 

 ignorance of Valentin's Essay), in a paper on the " Histology of the 

 Dermal Tissues," &c., in the British Journal of Dental Science, 1857. 



The coarse anatomy and relations of the Echinus-tooth are then 

 described, and the question is discussed as to how far the organ 

 resembles and how far it does not resemble the incisor tooth of a 

 Rodent mammal, to which it has constantly been likened. 



Some remarks then follow on the method of investigation, which 

 the peculiar physical characters of the structure render very difficult. 



Before describing the histology of the mature tooth, the author 

 premises some succinct remarks upon the several elementary parts 

 that are formed at its growing extremity, and by which its complex 

 structure is built up — showing how the shape and plan of these ele- 

 ments determine the microscopical appearances of the several regions 

 of the tooth as seen in different sections. 



These elementary parts are — ( 1 st) the Primary plates, which con- 

 sist of a double series of triangular sheets of calcareous matter, and 

 which constitute the physiological axis of the tooth, about which, and 

 connected with which, the four secondary elements are developed. 

 These latter are (2nd) the Secondary plates, lappets of similar cal- 

 careous sheets attached to the outer edge of the primary plates ; (3rd) 

 the Flabelliform processes, elaborate reticulations of calcareous fibres 

 ending in fan-shaped extremities ; (4th) the Keel fibres, certain long 

 cylindrical rods with club-shaped ends of the same chemical nature, 

 which pass towards the enteric region of the tooth in their growth ; 

 and (5th) the Enamel Rods, which are minute very short develop- 

 ments of the same character, and which are formed in the opposite 

 direction. Thus far a primary and secondary stage of formation are 

 represented : a third stage, that of consolidation, now occurs in the 

 development of (6th) the Soldering particles, multitudes of minute 

 disks of carbonate of lime which appear over the whole surface of the 

 previously-formed elementary parts, and by which they are soldered 

 together, the intervals between these (in a certain sense) constituting 

 the tubular character of the mature tissue. 



The primary plates, secondary plates, and the proximal portion of 

 the flabelliform processes are stated to constitute the body of the 

 tooth ; the distal extremities of the flabelliform processes the skirtings 

 of the enteric region of the body of the tooth ; the keel fibres wholly 

 form the keel ; while the short enamel rods compose the thin white 

 layer on the dorsal surface of the tooth — the enamel. 



The histology of the tooth is remarkable as exhibiting apparent 

 inconsistencies in difl'erent lines of section. A vertical section of the 

 tooth presents the appearance of vertebrate bone — lacunae, canaliculi, 

 and lamellae ; while a transverse section displays some regions resem- 

 bling dentine (the body of the tooth), and others having the closest 

 similitude to an oblique section of the shell of some Mollusca, such 

 as Pinna. 



The author then proceeds to describe in detail and with particularity 

 the form and progressive growth of the several elements of the tooth 



