359 
and Mr. Owen considers that the entire length of the tooth might 
be 34 inches, and the breadth at the basis 14. inch. 
The external longitudinal grooves, which correspond to the in- 
flected folds of the cement, extend) upwards from the base of the 
tooth to about three-fourths of its height, decreasing’in number as 
the tooth diminishes in thickness; and disappearing about half an 
inch from the summit of the tooth.» Each fold of cement penetrates 
less deeply as the groove approaches its termination ; .and Mr: Owen 
conceives that the structure of the upper part of » the tooth may be 
more simple than that of the lower, : but he has not pe been able 
to extend his investigations ‘to it. 
The dentine consists of -a slender, sar ek scanbeall schist or 
** modiolus,” hollow for a certain distance ‘from ‘its base, and -radi- 
ating outwards from its circumference a series of vertical -plates, 
which divide into two, once or twice, before they terminate at the 
periphery of the tooth. ~Each of these diverging and) dichotomizing 
vertical plates gives off throughout: its: course narrower: vertical 
plates, which stand at nearly right’angles to the main-plate; in rela- 
tion to which they are: generally opposite,-but sometimes alternate. 
Many of the secondary plates, which are given off near the céntre 
of the tooth, also divideinto two before they terminate. ‘They par- 
take of all the undulations which characterize thie inflected folds. of 
the cement. ©: 
The central pulp-cavityis reduced to: a lites about: the upper <fthind 
of the tooth;’ but fissures radiate from it, corresponding in number 
with the radiating plates of the dentine.’ One’ of these fissures is 
continued along’ the middle ofeach'plate, dividing where it divides, 
and penetrating each bifurcation and process ;, the main fissures ex- 
tend to-within a line or ‘halfia line of the periphery of thé tooth; 
the terminations of these, as well as the fissures: ofthe lateral pro+ 
cesses, suddenly dilating into subcircular, oval, or pyriform spaces. 
All these spaces constitute centres of radiation) of :the fine calcige- 
rous tubes, which; | with their uniting ‘clear substance, constitute 
the dentine!’ The numberof these calcigerous tubes; which are the 
centres of minor ramifications, defies all calculations. Their diameter 
is the roooth of a ame; with ai tote acres te seven diameters of 
their cavitiess: 9) => ibeo W ot 3 
» Mry Owen then compares dhe Senegtaine of the’ ‘section: of a écioth 
procured)in the sandstone:of Coton-End Quarry, and lent to;him by 
Dr. Lloyd of ‘Leamington. ‘The itooth nearly resembles in ssize-and 
form the smaller teeth:of Labyrinthodon figured by Prof. Jager. All 
the peculiarities of the labyrinthic'structure of the Keuper tooth are 
so ‘clearly preserved ‘in this’ cee al ae niffeiionests are: jade 
of! a specific nature: 
‘At the upper-part.of the tooth a tity lagen a cAnaiivedt besides a 
sesetinte of cement; is inflected’at each groove towards: the centre,of 
the dentine ; but about’ the middle of the tooth the’ enamel) disap- 
* Mr. Owen has subsequently ascertained that this is not true enamel, 
but a layer of firm dentine, separated’ from the test bya ee stratum of 
fine calcigerous cells. Ns 
202 
