THE DENTAL TISSUES OF THE ORDER RODENTIA. 
539 
part of the tooth, in the terminal fifth of their course, form a rich plexus of branches, 
and finally terminate in minute irregular cells. Then comes a thin investment of trans- 
parent cementum, which terminates at the margin of the enamel. The dentine is 
dotted throughout with fine cellular markings, which seem to indicate the form of 
the cells of the dentinal pulp previous to its calcification, and give a coarse appearance 
to the tissue. The dental tissues of this tooth are not distinguishable from those of the 
next species. 
Arctomys 'pruinosus, Gmel. (Zoological Society). — In an upper incisor the enamel 
averages the 75th, and the dentine the 5th of an inch ; as the dentine closely resem- 
bles that of the Quebec Marmot, the description need not be repeated. The enamel ex- 
hibits a considerable departure from that of the Sciuridse, though not wholly different 
in type of structure. The fibres in the first part of their course are arranged in 
parallel layers, which have a thickness of about the 4580th of an inch, as seen in a 
longitudinal section and illustrated in fig. 12 E. The layers lie at right angles with 
the surface of the dentine, and extend across the inner two-fifths of the enamel, at 
which point the uniform lamelliform arrangement is broken up by a change in the 
direction of the component fibres. In a transverse section of the tooth the component 
fibres of the enamel layers are seen arranged in a single series, lying side by side, and 
crossing those of the adjoining layers at an angle so as to form a diamond pattern 
over the inner part of the tissue (fig. 13). The decussating fibres of the superimposed 
layers, after traversing the inner two-fifths of the enamel, change their direction, be- 
come more parallel, and in waved course advance upwards and outwards till they 
reach the surface of the tooth. But although the course is much more parallel in 
the outer three-fifths than in the inner two-fifths of the enamel, yet if the focus of 
the instrument be carefully changed, alternate layers of fibres may be seen crossing 
each other at an angle to form very elongated and irregular diamond- shaped figures. 
The line at which the change of direction takes place is not definitely marked as in 
the Sciuridse and many other rodents. In a longitudinal section, it will be seen that 
the fibres in the terminal part of their course are directed upwards at an angle of 
70° with the surface of the dentine, and that small cells are scattered through this 
part of the enamel. In addition to these minute irregularly disposed cells, lines of 
cells may be seen commencing at the surface of the dentine, pursuing a curved course, 
and finally crop out at the surface of the enamel : I have not seen similar out-cropping 
lines of cells in the enamel of any other rodential teeth. The enamel fibres may in 
a transverse section be traced through the coloured portion of the enamel to the sur- 
face of the tooth. 
Castor Jiher, Linn. (Zoological Society). — In an upper incisor the enamel is about 
the 100th and the dentine the 5th of an inch in thickness. 
It has been observed by Professor Owen, that vascular canals are continued from 
the pulp-cavity a short distance into the dentine. In the specimen before me, the 
middle third of the dentine is traversed by vascular canals, which turn short upon 
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