THE DENTAL TISSUES OF THE ORDER RODENTIA. 
537 
the tooth no transverse marking can be traced. If a section be made obliquely in 
the vertical plane of the tooth, so as to cut the fibres of one set of layers in their 
length and the others transversely, vve shall have straight fibres with intervening rows 
of fibres cut across (fig. 8 E). If the section be taken from the protruded portion of 
the tooth, the cut extremities will be nearly square ; but if it be taken from near the 
base, where the enamel has not attained its full solidity, they will have a less regular 
outline. Sections of this kind show that the fibres are a little longer than broad, and 
that the longer axis is placed in the length of the tooth. 
A vertical section will however show most strongly the peculiarities that belong to 
this family of rodents, namely, the small relative amount of enamel, the uniform cha- 
racter of the layers, the uniform lines that mark their junction, and their straight and 
rectangular course outwards from the surface of the dentine, together with the angle 
at which they are bent in the external portion of the enamel. 
An incisor in which these several conditions of the enamel are found to exist, may 
I think be safely pronounced to belong to a species included in the family Sciuridse, 
and in all probability a member of the genus Sciurus or Pteromys. 
The molar teeth of S. niger present no sufficient peculiarity in the structure of the 
dentine to render a description necessary. Neither can I find any point of difference 
worthy of notice in the corresponding teeth of S. vulgaris, capistratus or cinereus. In 
S. erythropus the dentinal tubes are continued, the 1500th of an inch, into the enamel, 
and in this short course branch in the manner shown in fig. 1 1 E. I have not found 
this peculiarity in any other squirrel which has come under my notice. 
The cementum of the molar teeth is not very abundant, even at the extremity of 
the fang; I cannot discover that it is continued over the surface of the enamel. If 
a vertical section of a molar tooth from either of the species I have named be care- 
fully examined, it will be seen that the cementum, where it commences in a thin layer, 
at the neck of the tooth is composed of uniform rods, directed from without inwards 
and a little downwards. When this tissue is thicker the rods are seen near the sur- 
face, but are lost amongst the lacunae and their canaliculi. The cemental rods are 
subgranular in structure, and exceed the enamel fibres in dimension. They average 
the 3450th of an inch in diameter, and are portrayed in fig. 10 C. I am not aware 
that this character of the cementum has been previously noticed, it is not however 
confined to the molars of the Sciuridse. 
In the molar teeth of Squirrels the enamel is far less peculiar than in the incisors, 
and offers no strongly marked characters by which the teeth can be recognized. The 
fibres are less regular in form, less clear and transparent, and less free from minute cells 
than in the front teeth ; neither have we a terminal portion taking suddenly an altered 
direction; nor is there any evidence that the fibres are arranged in parallel layers 
transverse to the long axis of the tooth. Their course is however, throughout, more 
or less waved ; and although many cross each other, yet all do not ; and when they do, 
the definite and constant angle preserved in the incisors is not observed in the molar 
MDCCCL. 3 z 
