560 
MR. TOMES ON THE STRUCTURE OF 
The uniform presence and more constant breadth of the inner fibrous portion of the 
enamel is rather unusual in Hystricine teeth, while it is constant and exists in even 
a greater degree in the molars of the Hare. In this particular therefore the molars 
of the Helamys show a slight relation to those of the Leporidae. 
Leporidji:. — In this family of rodents my observations have been confined to 
members of the genus Lepvs. The incisors exhibit a type of enamel which I have seen 
in the teeth of no other rodents. This tissue is no longer divided into an outer and 
inner portion, with the component fibres arranged in lamellae in one and not in the 
other, and at different angles in the two parts ; but, on the contrary, the fibres without 
a lamelliform arrangement proceed with but slight flexures from the surface of the 
dentine to the outer surface of the enamel. Neither is the enamel dotted over with 
cells, as is that of Hystricine teeth in the lamelliform portion of the texture. 
Lepus timidus (Linn.). — In the Hare the dentine is permeated by vascular canals, 
both in the anterior and posterior half of the incisors ; but they are more numerous 
in the latter than in the former part. These canals become in the extruded portion 
of the tooth lined by a layer of dense non-tubuiar tissue, presenting the appearances 
delineated in fig. 51. The anterior enameled part of the incisors is bordered by a 
peripheral plexus of branches interspersed with a few branching cells, and the 
posterior half by a plexus of branches without cells. In a longitudinal section, such 
as that showm in fig. 50, the dentinal tubes from their commencement in all parts of 
the tooth branch, but in a transverse section the branches seem fatherless numerous ; 
hence it would appear that they extend principally in the long axis of the tooth. 
Prof. Owen has observed that “ the tubes which pass to the opposite or posterior 
surface of the tooth are less numerous, less parallel, and less closely packed together ; 
they send out more and larger branches, which decussate each other in an elegant 
arborescent manner*.” In addition to these peculiarities, the tubes are sensibly 
larger in the lateral and posterior than in the anterior part of the tooth. In a longi- 
tudinal section, such as that shown in fig. 50, the enamel fibres may be traced through 
the whole thickness of the tissue. Generally their course is straight, or nearly so, and 
at an angle of from 50° to 70° with the surface of the dentine, but in places the angle 
is varied, and the fibres are a little bent in the one or other direction. The tissue is 
rendered rather unusually transparent by tlie close union of the component fibres, 
and its comparative freedom from small cells. 
A corresponding section from an upper incisor differs from the one already described 
principally in the diminished thickness of the enamel. The enamel fibres have a 
diameter of about the 6800th of an inch. 
The molar teeth of the Hare have dentine, whicli is traversed by vascular canals, 
that extend from the pulp-cavity to near the peripheral surface of the tissue. Previous 
to the part coming into wear the canals are lined with a non-tubular tissue, and ulti- 
mately become almost or quite obliterated. The dentinal tubes are sensibly larger, 
* Odontography, page 405. 
