546 
MR. TOMES ON THE STRUCTURE OF 
15,000th of an inch, which dimensions are retained after they have passed with that 
tissue. The tubes pass onwards with slight tortuosities in a line with the direction 
of the enamel fibres, until they reach the outer third, when they are lost or turn 
suddenly at a right angle, and after advancing a short distance downwards, disappear. 
These conditions are shown in fig. 29. The fibres of the enamel show no disposition 
towards a lamelliform arrangement, excepting near the terminal edge, where the 
lamellse have, in a longitudinal section, serrated margins. The fibres leave the sur- 
face of the dentine on the sides of the tooth at an angle of about 50°, and in the outer 
third of their course come a little outwards. In the depression on the masticating 
surface, they proceed at a right angle from the surface of the dentine. 
The molars of the Jerboa, though like those of marsupial animals in having the 
dentinal tubes continued into the enamel, differ from them in having serrated lamellae 
in the enamel near its terminal edge, and hence may be distinguished both as not 
belonging to the marsupial order, and as belonging to the Muridas. 
I have been obliged to make upwards of twenty sections of the teeth of this in- 
teresting animal before I could fully satisfy myself on the various points of the dental 
structures ; indeed in the early part of the investigation I despaired of making out the 
arrangement of the component fibres of the enamel. 
The next animal on the list is Pedetes Cafer, but as the teeth resemble more closely 
the teeth of the Flystricidae than any other group of animals, and are altogether dis- 
similar to those of the Muridee, I shall postpone the description till I have gone 
through my specimens of Hystricine rodents. 
The teeth which have come within my reach from members of the following genera, 
Mus, Hapalotis, Gerhillus, Hydromys, Hesperomys, Arvicola and hemmus, so closely 
resemble each other in the general characters of the structure of the dental tissues, 
that it will be necessary to describe minutely those of a typical species only, and 
afterwards to note any specific differences that are found in teeth from other 
members of these genera. I will therefore take the teeth of the common Rat {Mus 
decumanus) as being typical of the family, and also on account of the ease with which 
the teeth may be obtained by any subsequent observer. 
Mus decumanus. — From a longitudinal section of a lower incisor, we learn that the 
dentine presents no specific or generic peculiarities. The dentinal tubes from their 
commencement give off numerous minute pilose branches, and terminate near the 
surface of the dentine by forming’ a dense plexus of minute ramifications. Their 
course is straight, or nearly so, excepting the secondary undulation, which exists in a 
greater or less degree in almost every specimen of dentine that has come under my 
observation. The tubes are oval in their transverse section, having a short diameter 
of about the 15,000th of an inch, and a long diameter which is placed in the length 
of the tooth, and averages the 10,000th of an inch. 
In the anterior part of the tooth the tubes gradually diminish in diameter from 
their commencement, while those in the posterior part retain their original size till 
