THE DENTAL TISSUES OF THE ORDER RODENTIA. 
541 
distinction between the two layers of enamel is still more obvious : the fibres of the 
inner half, being cut across, give the appearance of fine decussation, oblique lines ; 
while those of the outer half run transversely to the surface, and are crossed by traces 
of concentric layers*,” That the appearance of decussation here mentioned is due to 
the crossing of fibres, and not to the direction of the section, is proved beyond doubt 
by taking a thin transverse section and breaking it across the centre, when the fibres 
of the layers included in the section may be viewed projecting from the broken edges ; 
or it may be even more distinctly demonstrated by removing a little of the partially 
calcified enamel from the lower portion of the tooth, and placing it with a drop of water 
between two pieces of glass. 
In the dentine of the molar teeth of the Beaver, I can find no characteristic pecu- 
liarity that needs description. The root of the tooth is composed partly of cementum, 
into which the dentine graduates, and through which vascular canals lead to the pulp- 
cavity. 
The fibres of the enamel correspond in arrangement with the external portion of those 
which form the enamel of the incisor teeth, and like them have a greater and less 
diameter, the former of which is placed in the length of the tooth. 
The enamel fibres are usually directed upwards at an angle of 30° with the surface 
of the dentine, and near the surface curve a little outwards ; but they may be found 
in some parts of the tooth making two slight curves before arriving at the surface. 
Near the upper part of the tooth the fibres are so closely united, that the enamel 
in places seems almost structureless ; but toward the roots, and where it is reflected 
into the depressions, the fibres are sufficiently distinct to be examined and measured. 
The enamel, where it attains its greatest thickness, is marked by oblique lines, 
which proceed from within outwards and speedily crop out on the surface. They 
appear to result from a difference of density rather than from the presence of minute 
cells, such as are found in the incisors of the Marmots. 
The cementum of the molars is granular, plentifully supplied with lacunee, and 
arranged in concentric laminae round the vascular canals and the roots of the tooth ; 
here and there a slight tendency to the arrangement in rods, similar to that of the 
Sciuridae, may be observed. 
Spalax typhlus, Pall. (Zoological Society). — This animal, though placed by Mr. 
Waterhouse in a section between Murina and ArvicoUna in the family Murids, 
resembles the Beaver in the structure of the enamel more closely than any other 
animal, which circumstance offers a sufficient reason for describing the dental tissue 
in this place. 
In an upper incisor the dentinal tubes present an oval transverse section with the 
long diameter placed in the long axis of the tooth. The long diameter attains the 
5000th, while the short diameter does not exceed the 10,000th of an inch. In their 
passage outwards they describe a sigmoid curve, with the general direction a little 
* Odontography, page 407. 
