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Sir. tOxMes on the structure of 
It is the purpose of the present communication to lay before the Society results 
obtained from an examination of the teeth of various members of the order Rodentia. 
I have had the opportunities necessary for extended researches in this division of 
Odontography, partly through the assistance of numerous friends, but principally 
through the liberality of the Council of the Zoological Society, who granted me the 
privilege of examining teeth from the duplicate specimens of their large collection of 
skulls. At the time I commenced the investigation, there seemed but little hope of 
finding any strongly marked and characteristic differences of structure in the dental 
tissues of the several families of this order of quadrupeds. The teeth of many rodents 
had already been submitted to the microscope, and the results published*. I had 
not proceeded far, however, in the investigation of this highly interesting subject, 
before it became apparent to me that the family Hystricidse and the Sect.Bathyergina of 
Waterhouse have a constant and exclusive character in the structure of the enamel ; 
that the Sciuridse have another character ; that the first and second sections of the 
family Murids possess a third ; and that the remaining sections of that family possess a 
fourth well-marked charaeter, and the Leporidae a fifth. I am told by Mr. Waterhouse 
that these results are of great importance, as affording evidence on the position of 
several species whose place in the order has not been definitely fixed, either by their 
external characters or by the structure of the skull, and hence they have been variously 
placed by naturalists, and even by the same author at different times. 
Before entering on a description of the structural characters that pertain to the 
teeth of the rodential families, or of individual teeth, it will be desirable to state those 
conditions which are common to the whole order, otherwise it would be necessary to 
repeat frequently the same fact. It has long been known that the incisors of Rodentia 
have the property of unlimited growth, and that the rate of growth equals the rate 
of loss by wear ; hence the exposed portion of the tooth is, in the normal state of the 
dental apparatus, maintained of uniform length. The pulp-cavity in a longitudinal 
section of the tooth is irregularly conical or wedge-shaped, and in a transverse section 
corresponds in some measure with the outline of the tooth. The dentinal tubes pass 
from every part of the cavity outwards and upwards towards the surface in more 
or less curved lines. 
The tubes which proceed from the cavity near the base of the tooth, are in many 
cases perceptibly larger than those that are situated higher up ; hence it follows, 
that, as the latter were once near the base of the tooth, the dentinal tubes undergo 
a diminution of calibre after their formation. In the teeth of the Sciuridse, I have 
found a difference of size amounting to a third or half between the tubes near the 
base and those near the surface, in wear. Professor Owen has observed that the tubes 
in the posterior or lingual half give off larger and more numerous branches than those 
constituting the anterior half of the tooth. My own observations lead to the con- 
clusion, that this difference does not always exist near the surface of the pulp-cavity, 
* Odontography, Prof. Erdl. 
