408 
MR. TOMES ON THE STRUCTURE OF 
mosing plexus of tubes, from which many pass to the cells of the cement ; hence 
these two tissues are not separated from each other by a strongly-marked line of 
demarcation, such as obtains in some other teeth of marsupial animals. 
Petaiiriis taguanoides. — The molar teeth of this interesting animal, though pos- 
sessing the marsupial characteristic strongly marked, yet are so different under the 
microscope from those I have already described as to be recognised at first sight. 
The dentinal tubes at their commencement at the pulp-cavity have a diameter not 
exceeding the of an inch. They follow a slightly undulating course to about 
two-thirds of their length ; they then make one large curve, the concavity of which is 
usually directed towards the crown of the tooth. At no part of their course do the 
dentinal tubes of the crown give off branches until they arrive at the enamel ; then a 
few, and a few only, divide into two branches, and are continued into that tissue. 
The tubes, on entering the enamel, make a small curve downwards, corresponding in 
direction to that in the dentine just described; they then follow an upward course, and 
are lost near the surface. The tubes usually suffer no change of dimensions on passing 
into the enamel, but generally keep a diameter of about e b.oo ' o lh of an inch, till they 
are lost in small opaque markings that are common in the enamel near its surface, 
Plate XXXVI. fig. 7- 
The dentinal tubes of the fangs of these teeth are perceptibly larger than those of 
the crown, and very commonly have a ragged outline, or are composed of lines of 
cells. They give off but comparatively few branehes, and these only near the surface 
of the fang. 
Ill the incisor teeth the dentinal tubes branch more abundantly than in the molars, 
and are less frequently continued into the enamel. 
In the fangs of the lower incisors the tubes are distinctly oval in section, having a 
greater diameter of y^xo lesser of rs^oir- The final large curve also has its 
convexity directed towards the crown of the tooth. 
Petaurus sciureus. — The structural characters of the teeth of this animal closely 
resemble those of P. taguanoides. There are however differences by which, on compari- 
son, the one may be known from the other. The dentinal tubes in their course from 
the pulp-cavity towards the crown arch with the convexity directed upwards; then 
again they give off numerous fine branches during the latter third of their course. 
In the fangs they dichotomize during the latter half of their length, but do not form 
the rich plexuses of tubes common in the Kangaroos ; neither do the branches go off 
at a right angle ; on the contrary, they follow much the same course as the parent 
tube. The fang is plentifully supplied with cement, which contains a few cells of very 
elongated form, with the long axis in the direction of that of the tooth. In addition 
to these there are great numbers of tubes placed parallel with each other, and at right 
angles with the surface of the tooth. 
Dasyurus ursinus. — The dentinal tubes of the crown of the tooth advance at a right 
angle with the surface of the pulp-cavity. Those which form the upper part of the 
