406 
MR. TOMES ON THE STRUCTURE OF 
tubes ; these recurve and present their convexities towards the outer surface of the 
tooth. 
Near the surface the dentinal tubes break up into an infinite number of delicate 
branches, resembling tufts of fine moss, effecting at the same part innumerable ana- 
stomoses. At and near the extremity of the fang, shoi’t branching dentinal tubes are 
interspersed with cemental cells, but higher up, towards the crown of the tooth, the 
cement and dentine are separated by a well-defined line of demarcation, across which 
a few only of the tubes advance to join the branches of the neighbouring cemental 
cells. 
Near the neck of the tooth the cemental cells are often altogether wanting in 
branches, and approach a circular form. 
Hypsiprymniis minor and pemcillatus. — The teeth of these animals resemble very 
closely in structure those of the Kangaroos. In the incisors, the enamel being less 
thick, present but one curve, and that in the direction of the axis of the tooth. 
In Hyp. penicillatus, the dentinal tubes, on entering the enamel, dilate into irre- 
gularly oval or circular cells, measuring about i 0,000 th of ao inch in diameter ; they 
then suddenly contract to g oiVoo th, and follow the course of the enamel fibres to near 
the surface ; then some few bend to a right angle with their original course and ter- 
minate in a point, their terminal course being directed towards the root of the tooth, 
fig. 4, The fibres of the enamel measure 7 smoo th of an inch in diameter. 
The dentinal tubes at their commencement at the pulp-cavity measure the 12000 ^^^ 
and at the point where they enter the enamel the — 4 — -th of an inch. 
In Hypsiprymnus minor, the dentinal tubes, on entering the enamel, often do not 
dilate at all, and when they do, the dilatation is continued for some little distance 
with an irregular outline. Hence, on accurate comparison, the teeth of the one 
species of Hypsiprymnus may be distinguished from those of the other. 
Phascolomys Womhat. — The teeth of this interesting marsupial are remarkable for 
the complete absence of tubes in the enamel. The dentinal tubes cease at the line 
of junction of the dentine with the enamel, and in no case pass across into the enamel. 
The fil)res of the latter texture are clear and transparent ; and arising from the peri- 
pheral surface of the dentine, after presenting one or two slight undulations, arrive 
at the surface of the enamel, which in the incisor teeth is invested with a thin layer 
of cement. The enamel fibres measure about the yyg-oth of an inch in diameter. 
Their course is not, as regards each other, everywhere parallel ; on the contrary, at 
certain points intervals will be left by their divergence, which are filled up by sup- 
plementary fibres ; then again bundles of fibres converge and cross each other. I 
have failed to observe the transverse striae mentioned by Prof. Owen*. The fibres 
have sometimes a slightly irregular outline, which gives an appearance of unequal 
dimensions to different parts of the same fibre, and not unfrequently longitudinal 
central lines may be seen, like the persistent nuclei of the developmental cells. 
* Odontography, page 398. 
