316 PROCEEDINGS OF SOCIETIES. 



lu 1841 A^alentiu, in Agassiz's 'Monograph on the Echino- 

 clerms ' (Anatovtie des Eclimodermes) , published a clescriijtion and 

 many good figures of the minute anatomy and growth of the 

 Echinus-tooth. 



Professor Quekett. in his 'Lectures on Histology' (1854), re- 

 ferring to the mini.^e mature anatomy of the organ, states its 

 ultimate structure to resemble bone and dentine of verfcebrata. 



Dr. Carpenter, in his work ' On the Microscope,' speaks of the 

 tissue of the tooth as essentially of the same nature as the shell 

 of the Echinidae generally (1856). 



Lastly, Professor W. C. AVilliamson describes the subject more 

 fully 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 

 ])remises some succinct remarks upon the several elementary parts 

 that are formed at its growing extremity, and by which its com- 

 plex structure is built up — showing how the shape and plan of 

 these elements determine the microscopical appearances of the 

 several regions of the tooth as seen in different sections. 



These elementary parts are — (1st) the Primari/ plates, which 

 consist of a double series of triangular sheets of calcareous 

 matter, and which constitiite the phj'siological axis of the tooth, 

 about which and connected with which the four secondaiy elements 

 are developed. These latter are (2nd) the Seconchin/ plates, lap- 

 pets of similar calcareous sheets attached to the outer edge of 

 the primary plates ; (3rd) the FlaheUiform 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 groAvth ; and (oth) the Haamel 

 Hods, which are minute, very shoi-t developments 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 develop- 

 ment of (6th) the Solderinr/ p)articles, multitudes of minute discs 

 of carbonate of lime Avhich 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 cliaracter of the mature tissue. 



