362 Transactions.—Geology. 
The pulp-cavity is filled, in the fossil state, by a deposit of the matrix. In 
one of the specimens a crack passes through the tooth into the cavitas. pulpi, 
and has become filled, along with the latter, with a homogeneous deposit. 
The thin crusta petrosa also consists of nearly diaphanous corpuscles, with 
numerous traces of partly effaced tubuli, so as to give ample assurance that 
this investing coat, like the rest of the tooth, is built up of caleygerous 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. А 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—erusta petrosa, enamel, and dentine—each of these. being 
merely modifications of continuous caleygerous 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 petrosa (or cement) appear to be directly 
continued from the tubuli of the ivory, although Retzius had expressly denied 
the continuation ; and again, in his remarks on the fossil teeth of the M astodon, 
he states that the minute terminations of the caleygerous tubes of the ivory 
are directly continued into the system of fine parallel tubes of the cement 
(crusta petrosa). 
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 
caleygerous tubes, as in the fossil teeth of the elephant. Owen alludes to 
this in his description of the tooth of the Leiodon, and states that the 
concentric arrangement of the lamell, 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 tubuli. 
A vertical section exhibits a pseudo-cellular arrangement. The edges of 
the caleygerous 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 appearance, which I have 
attempted to represent in Pl. X XIV. F, shows itself. 
