362 Transactions. — Geology. 



The pulp-cavity is filled, in the fossil state, by a deposit of tlie matrix. Tn 

 one of the specimens a crack passes through the tooth into the cavitas 2^ul2n, 

 and has become filled, along with the latter, with a homogeneous deposit. 



The thin crusta 2yetrosa also consists of nearly diaphanous corpuscles, with 

 numei'ous traces of partly effticed tubuli, so as to give ample assui-ance that 

 this investing coat, like the rest of the tooth, is built wp of calcygerous tubuli 

 and calcified deposits ; but, that the tubuli are no longer continuous with those 

 of the dentine, or have any relation to the nutrition of the tooth — merely, in 

 fact, serving as an inert, hardened crust, to protect the living dentine beneath. 



There is no clear line of demarcation between what has been called the 

 enamel and the dentine. It cannot be seen where the one begins and the 

 other terminates. A microscopic examination shows that the tubuli (which 

 do not exceed in diameter "0002 inch), together with the calcareous deposit in 

 the intertubular spaces (averaging -0012 inch), form the substance of every 

 part of the tooth — crusta petrosa, enamel, and dentine — each of these being 

 merely modifications of continuous calcygerous tubes and bony corpuscles, of 

 which only the crust has lost the properties of living matter. Owen must 

 entertain the same view, although he is evidently guarded in his expressions. 

 In his remarks on the microscopic examination of the teeth of the elephant, he 

 notices that the tubuli of the crusta petrusa, (or cement) appear to be directly 

 continued from the tubuli of the ivory, although Retzins had expressly denied 

 the continuation ; and again, in his remarks on the fossil teeth of the Mastodon, 

 he states that the minute terminations of the calcygerous tubes of the ivory 

 are directly continued into the system of fine parallel tubes of the cement 

 (crusta 2^etrosa). 



Towards the centre of the teeth, in a transverse section, concentric contour 

 lines are seen, caused by an opaqueness of the intertubular deposit (F). 

 Separation of the fossil tooth into superimposed layers takes place along 

 these more opaque portions of the dentine, directly across the course of the 

 calcygei'ous tubes, as in the fossil teeth of the elephant. Owen alludes to 

 this in his desci'iption of the tooth of the Leiodon, and states that the 

 concentric arrangement of the lamellfe, arising from the decomposition of the 

 tooth, has been used as an argument in support of the untenable " excretion " 

 theory. 



The iridescent lustre of the polished surface of the transverse section 

 is owing to the play of light on the reflecting walls of the radiating tabuli. 



A vertical section exhibits a pseudo-cellular arrangement. The edges of 

 the calcygerous tubes refract the light in short, brilliant lines, or appear as 

 dark bars, about -001 inch in length. Where the section passes through the 

 tubuli in a more or less oblique direction, a singular apijearance, which I ha\'e 

 attempted to represent in PI. XXI\^. F, shows itself. 



