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XXVIII. On the Structure of the Dental Tissues of the Order Rodentia. 
John Tomes, Surgeon-Dentist to the Middlesex Hospital. 
Communicated hy William Bowman, Esq., F.R.S. 
Received February 19, — Read May 30, 1850. 
In a memoir on the Structure of the Dental Tissues of Marsupial Animals, printed 
in the second Part of the Philosophical Transactions for 1849, I pointed out certain 
peculiarities in the structure of the enamel common, with one known exception only, 
throughout that order of quadrupeds, and found in other mammalian teeth in a few 
isolated cases only*. 
* Having in a former paper^ stated that the continuation of the dentinal tubes into 
the enamel appears to be a constant character in the teeth of marsupial animals, 
excepting only in those of the Wombat, I can now add that I have found it to 
hold good in many other members of the families from which I have already given 
examples ; and also in members of those families which are not mentioned in my 
paper ; and moreover, that further research has exposed no other exception to the 
rule than that which I have already cited. 
I find that in Macropus penicillatus most, if not all, of the coronal dentinal tubes 
are continued into the enamel, and in the latter part of their course are bent rec- 
tangularly downwards towards the fang of the tooth. In Halmaturus Derhianus the 
dentinal tubes are continued into the enamel, but are not subject to the terminal 
flexure observed in the preceding example. The dental tissues of the Dasyuriis viver- 
rinus closely resemble those of the Dasyuri already described. The teeth of Didel- 
phis californlca and Didelphis cancrivora, approach very closely in structure to those 
of the Didelphis virginiana. 
I am indebted to Mr. Gould for opportunities of examining the teeth of Myrme- 
cobius fasciatus, Perameles nasuta and Chccropus. In each of these creatures the 
dentinal tubes are continued into the enamel. In Phascolarctos fuscus, the dentinal 
tubes that proceed towards the tubercles of the teeth are continued in considerable 
numbers into the enamel; but on the sides of the teeth their continuation is less 
frequent. Here the enamel fibres are more strongly marked, and larger than in any 
other marsupial tooth that I have examined. 
Several specimens of fossil marsupial teeth have been examined, and are found to 
correspond in structure with those of the recent species, to which the fossil ones are 
most nearly related. 
November 24, 1849. 
MDCCCL. 
t Philosophical Transactions, Part IL for 1549. 
3 Y 
