THE DENTAL TISSUES OF THE ORDER RODENTIA. 
549 
frequency and strength of their transverse markings. In a longitudinal section of a 
lower incisor the layers leave the dentine at an angle of 40°, and have a thickness of 
about the 7500th of an inch. The whole thickness of the enamel is about the Y-|- | oths 
of an inch, of which xi occupied by the lamelliform, and jsVoths by the outer 
or fibrous portion of the enamel. The latter lies at an angle of 15° with the surface 
of the dentine. 
It will be seen, on referring to the description of the corresponding teeth and sec- 
tions of Mus decumanus, that the angle at which the laminae of enamel fibres leave 
the surface of the dentine, is sufficiently different from that in the Hapalotes to 
distinguish the teeth of these creatures from each other. 
The molars of the Hapalotes resemble those of the Rat. I am indebted to the 
kindness of Mr. Gould for an opportunity of examining the teeth of these animals. 
Gerhilliis Shawii (Devern.) stands next on my list. The dentine of the incisors of 
this small rodent is peculiar in having a few vascular canals extended from the pulp- 
cavity a short distance into its substance, which in a transverse section gives an 
uneven outline to the surface of the pulp-cavity. The dentinal tubes give off branches 
throughout the whole of their course, are interspersed with small cells, and are sub- 
ject to irregular secondary undulations ; in addition to which, the whole substance 
of the dentine has a cellular appearance as though the developmental cells had re- 
tained their outline during the process of calcification, instead of becoming confluent 
and homogeneous. 
In a longitudinal section the laminae leave the surface of the dentine at an angle 
of 55°, and curve upwards and outwards through about xsVoliis of an inch : they 
then give place to the fibrous portion in which the fibres curve upwards and outwards, 
their general course being at an angle of 20° with the surface of the dentine. 
The laminae are bordered by blunt serrations, and are subject to transverse markings, 
in both particulars resembling the enamel of the corresponding teeth of the Hapalotes 
more closely than that of the Rat. 
In a transverse section the enamel of the incisors could not be distinguished from 
that in the Rat. 
In the molar teeth the dentinal tubes are continued into the enamel the 1500th 
of an inch. In no part of the latter tissue could I distinguish a lamelliform arrange- 
ment in the enamel. 
The teeth of Hydromys chrysogaster of New South Wales resemble those of the 
Rat. The incisors of the Hamster, Cricetus fumenta, lus (Pall.), can scarcely be 
distinguished from those of Mus decumanus. 
Hesperomys Darwinii (Waterhouse) possess teeth so like in minute structure to 
those of Mus decumanus that a special description is unnecessary. 
Geomys umhrinus (Rich.), though not placed by Mr. Waterhouse in the family 
Muridse, have incisors so Rat-like, that it would require the presence of sections of 
the teeth of this creature and of the Rat to distinguish the one from the other, 
and even then there would be some difficulty in finding characteristic differences 
