548 
MR. TOMES ON THE STRUCTURE OF 
the component fibres is so perfect that all appearance of structure is lost. This ob- 
servation applies equally to other scalpriform teeth. The coloured surface pre- 
sents the character of a stain, through which the fibres are continued. If the section 
be tolerably thick and a little oblique, the two cut edges will give the appearance of 
a coloured lamina. But the structure of the enamel may be demonstrated, though 
in a less complete manner, by taking a little of the soft partially developed tissue 
from the base of the tooth, and with a little water placing it between two slips of 
glass. The decussation of the fibres will be then shown, and some will present a 
beaded outline ; small portions may also be found in which development is more 
forward, and some of the fibres armed with small lateral processes will appear in 
the field of the microscope. Some of these appearances are shown in fig. 33. 
The upper have a somewhat thicker coat of enamel than the lower incisors ; it 
averages about the 2ilch of an inch, of which the lamelliform portion occupies about 
the 333rd and the outer or fibrous portion the 666th of an inch. 
In a longitudinal section the lamellse leave the surface of the dentine at an angle 
of 55°, which is a little wider than that formed by the corresponding parts in the 
lower incisors. The layers describe a gentle curve upwards and outwards, and the 
margins are serrated through the greater part of their course, but less strongly than 
in the corresponding lower teeth; in addition to which the layers are frequently 
marked by transverse lines. Towards the outer part of their course, the serrations 
become faint and ultimately give place to a smooth outline. The fibres in the outer 
part of the enamel run at an angle of 25° with the surface of the dentine. 
In the molar teeth the dentine presents no generic peculiarity, neither does the 
enamel about the cusps of the teeth, but at and near its terminal edge on the neck of 
the tooth, the lamelliform arrangement with the serrated edges holds good, both in 
the Rat and in other rooted molars of the genera Mus, Hapalotes, Hydromys and 
Hesperomys. The cementum about the neck of the tooth is arranged in rods, similar 
to those already described in the corresponding teeth of the Squirrels. 
The teeth of Miis rattus present no structural peculiarities by which they can be 
distinguished from M. deciimamis, neither do those of the following rodents, excepting 
in size : — Mus Alexand, hms, sylvaticus, miisculiis, minutus fusipes. In examining 
the molar teeth of Miis giganteus, I find a peculiarity which may perhaps be specific. 
The fibrous enamel on one side of the cusps is separated from the dentine by a layer 
of perfectly transparent and apparently structureless enamel, which is thickest near 
the apices of the cusps, and gradually thins tiit it is lost at the bottom of the fissures. 
On the sides of the tooth the serrated laminte of the enamel are strongly marked, 
and extend through a larger portion of the tissue than is common in the corresponding 
teeth of any other rat which I have examined. 
In the teeth of Hapalotes alhipes and longicaudatus of New South Wales, I find a 
very close structural lesemblance to those of the Rat, the principal difference being 
in the more blunted shape of the serrations of the enamel lamellm, and the greater 
