532 
MR. TOMES ON THE STRUCTURE OF 
In those rodents whose molars are of persistent growth, whether the tooth is com- 
posed of denticles or confluent denticles, the dentinal tubes of that part of the tooth 
which is protruded and exposed to wear, are cut off from their connection with the 
pulp-cavity in the same manner as in the incisors. This beautiful provision of nature 
for rendering solid and extra-vascular parts that are about to be exposed to mecha 
nical abrasion from external objects, is not confined to the teeth of rodents, or to 
dental tissues alone. 
The Orycteropus has teeth which are permeated by a series of canals, which take 
a parallel course at tolerably regular intervals in the length of the tooth. PTom each 
canal a system of dentinal tubes radiates, the terminal branches of which inosculate 
with corresponding ones from neighbouring systems. In the protruded portion of 
the tooth, the medullary canals are rendered solid by the development of a clear non- 
tubular tissue, whereby the once open extremities of the tubes are closed (fig. 2). 
The teeth of reptiles of the genus Varanus are at their middle part and base com- 
posed of a series of dentinal systems, as are those of the Labyrinthodon* (fig. 3). In 
parts, in the same manner as the Haversian systems do in bone. But there are manj" instances in ■«’hich parallel 
dentinal systems are united to each other, throughout their whole length, by a thin longitudinal lamina of den- 
tine, the tubes of which belong as much to the one as to the other system, while the free parts of each system 
are coated with enamel, and this with cement. The molar teeth of the Water-Rat and Ondatra offer good ex- 
amples. It is desirable to have a term to express this condition ; confluent denticles w’ould, I think, answer the 
purpose, and confluent dentinal systems where a like condition is observed, without the presence of enamel or 
cementum as a uniting medium. 
* It has been usual to describe the tooth of the Labyrinthodon as being divided into numerous compartments, 
by tortuous inflections of the cementum from the surface towards the centre. I believe it is admitted by 
w'riters on Odontography, that when enamel and cementum are present in the same tooth, the latter tissue 
holds a position external to the enamel. Professor Owen, in his paper on the teeth of the Labyrinthodon, 
printed in the Geological Transactions, says of the Ichthyosaurus, “ In this extinct Saurian the external layer of 
cement (for the enamel ceases at the base of the crown) is inflected at pretty regular distances around the cir- 
cumference of the tooth towards its centre ; ” then again, “ The plan and principle of the structure of the tooth 
of the Lab\ninthodon are the same as those of the tooth of the Ichthyosaurus, hut are carried out to the highest 
degree of complication.” But in a beautiful series of sections of the tooth of the Lahyrinthodon Jaegeri in the 
possession of Dr. Mantell (to whom I am indebted for their use), it is clearly shown that the division of the 
tooth into numerous compartments takes place within a general investment of enamel, external to which the 
cementum would be placed if it existed on this part of the tooth. 
The inflections of the cementum, observed by Professor Owen in the tooth of the Ichthyosaurus, and com- 
pared by him to the supposed inflection in the tooth of the Labyrinthodon, take place, as he observes, below the 
terminal line of the enamel; hence the two cases do not admit of comparison. Similar cells to those which 
occupy the line w'hich lie between the dentinal system and serve as a m.edium of connection between the den- 
tinal tubes of adjoining systems in the latter animal, are found in equal or even greater numbers near the 
periphery of the dentine, and within the enamel of very many teeth. Hence the presence of these cells in the 
too.th of the Labyrinthodon is not sufficient to prove the existence of cement between the dentinal systems, unless 
it is at the same time shown that they are external to the enamel, or that that tissue is absent. The tooth of 
the Labyrinthodon is in truth made up at its apex of a single dentinal system coated with enamel ; below, it is 
divided into numerous systems, which have a peculiar and characteristic outline and position. Each system 
usually coalesces at one part of its circumference with a neighbouring system, by a narrow vertical process of 
