550 
MR. TOMES ON THE STRUCTURE OF 
sufficiently well-marked to render the conclusion trustworthy if any doubts were 
thrown on the authenticity of either of the specimens. 
Arvicola ampMhius (Linn.). — It would be extremely difficult to point out the cha- 
racters by which sections of the incisor teeth of this creature could be distinguished 
from the corresponding ones of several teeth I have already described, especially those 
of the common Rat (M. decumanus). The serrated enamel lamellae leave the dentine 
at nearly the same angle ; the serrations are perhaps finer and less strongly marked, 
and the lamellae are more frequently crossed by equidistant transverse lines. The 
rootless molar teeth, however, are sufficiently different from those of the Rat. In a 
longitudinal section, it will be seen that on one side of each denticle the enamel is 
composed of an inner lamelliform portion, with the edges of the lamellae serrated as 
in the incisors, and another portion in which the enamel fibres are parallel ; while on 
the opposite and posterior surface of the denticles the lamellae are absent, and the 
enamel fibres pass across the structure in a curved line to the surface. In a transverse 
section, it is seen that the two conditions pass insensibly into each other at the bottom 
of the longitudinal grooves, where the component denticle coalesces, and also imme- 
diately behind the longitudinal ridges which mark the sides of the teeth. 
In the Field Vole, Arvicola nivalis (Martin), the teeth, both molars and incisors, 
structurady resemble those of the A. amphihius. 
The incisors of the Bank Vole, Arvicola glareohis (Schreb.), are rat-like ; but in the 
molar teeth the serrated lamelliform arrangement of the enamel is very indistinct. 
In the lower incisors the serrations of the lamellae are shallow, while the transverse 
markings are rather strong. The appearances presented in a longitudinal section are 
delineated in fig. 34. 
The incisors of Lemmus Norwegicus, Desm. (Zoological Society), do not offer any 
structural differences from the preceding group worthy of description. 
Fiber zibethicus (Linn.). — This animal is placed by Mr. Waterhouse in the family 
Murids, section Arvicolina. The dental tissues do not well accord with that posi- 
tion, but seem to indicate a nearer relation to the Beaver and Dormouse. In the 
molar teeth, however, the enamel in places resembles that in the corresponding organs 
of Arvicola amphibiv'' and A. nivalis, but the serrated lamelliform arrangement is not 
as uniform or as well-marked as in those animals. Yet Mr. Waterhouse’s arrange- 
ment of Rodentia is, with but few exceptions, so strongly corroborated by the struc- 
ture of the dental tissues, that in this case, where there is conflicting testimony in 
these tissues as to v/here they should be placed, I shall do well to describe them in 
the position he has assigned to the animal. 
In the upper incisors, a few vascular canals radiate from the pulp-cavity into the 
dentine, but they are far less numerous, and run a shorter course than in the corre- 
sponding teeth of the Beaver. The dentinal tubes resemble those of the latter animal, 
excepting that they terminate at the enamel, without the presence of a peripheral 
layer of cells. 
A longitudinal section exhibits the enamel lamellae passing outwards in an irre- 
