538 
MR. TOMES ON THE STRUCTURE OF 
teeth. A vertical section of a molar of S. niger exhibits the usual characters of the 
dental tissues in these teeth (fig. 9). In S. cinereus the enamel fibres are in a ver- 
tical section, seem to be minutely granular, well-marked, and subject to one or two 
gentle curves near the surface ; they are in the outer part of the tooth very strongly 
marked, from being highly granular, and less perfectly united laterally than in some 
other teeth. 
Tamias Lysteri, Rich. (Zoological Society). — I have been able to procure a lower 
incisor only of this creature. The structure of the tooth very closely resembles the 
incisors of squirrels : the dentine is similar, as are the enamel layers both in shape 
and arrangement ; and the component fibres of the contiguous layers cross each other 
at the same angle. The decussation however ceases, and the parallel arrangement is 
assumed in about the middle of the enamel ; this difference, if found constant in all 
the species, will serve to distinguish the Tamias from the Squirrels. 
Spermophilus (Zoological Society). — The structure in the incisors of this animal 
deviates a little from that in the Squirrels ; in thickness, the enamel, as seen in 
a transverse section, is about 300th and the dentine the 12th of an inch ; the dentine 
is much the same as in the genus Scmrus, excepting that the dentinal tubes of the 
anterior half of the tooth measure about the 7500th of an inch in diameter, while in 
the posterior half they average the 6000th of an inch : the enamel is different, and 
advances a step towards another type. The enamel layers incline upwards at an angle 
of 78°, instead of preserving the reetangular position, and they are less regular in their 
course, and less uniform in size than in the former genus. A transverse section shows 
that the component fibres of the adjoining layers decussate one another throughout the 
inner half of the enamel, and then become parallel. An oblique section in the long 
axis of the tooth will expose alternate straight fibres and rows of ends of fibres cut 
transversely ; and in the latter it may be observed that they have an oval rather than 
a square section, the long diameter of which is one-third greater than the short ; this 
is best seen near the cutting edge of the tooth. If the section be taken from near the 
base, the fibres in their transverse section are more circular, less compressed, and much 
less intimately united to their fellows, while at the opposite extremity of the tooth 
they are so closely connected, that in places the enamel seems a dense transparent 
structureless mass. 
The enamel layers in a vertical section have a thickness of about the 5000th of 
an inch. In a transverse section, the fibres have a diameter of about the 10,000th of 
an inch in their smaller, and the 5000th in their greater diameter. 
Arctomys Empetra, Schreb. (Zoological Society). — In a lower incisor the enamel 
has an average thickness of 75th of an inch, and the dentine the^th of an inch. The 
dentinal tubes have an average diameter of the 10,000th of an ineh : I do not find any 
difference in the dimensions of the dentinal tubes at the anterior and posterior parts 
of the incisor. They terminate at the enamel in a peripheral layer of minute irregular 
cells, much in the manner shown in fig. 12 and 13. Those directed towards the back 
