410 
MR. TOMES ON THE STRUCTURE OF 
bold curve outward, with the convexity directed upwards ; in the fang they make a 
second large curve with the concavity directed upwards. 
The tubes, when in the enamel, in addition to a multitude of minute undulations, 
describe one large curve with the concavity directed upwards. They for the most 
part rapidly diminish in size ; sometimes, however, they dilate into small cells, and 
again continue their course. Previous to terminating some few give a number of 
small branches from their convex side (fig. 10 B), which rapidly diminish in size and 
are lost. At about the point where the tubes are lost the enamel fibres become 
tolerably distinct, though with a somewhat ragged indefinite outline. Their diameter 
is about the 1 0,000 ^ of ioch ; and they take a tolerably direct course outwards. 
I have stated that the enamel fibres of the tooth of the Thylacinus resemble in a 
slight degree those of ordinary carnivora, but the one structure cannot for a moment 
be mistaken for the other, see figs. 10 and 11. Small cells are scattered through the 
enamel of the Thylacinus and near the surface in contour lines. 
The dentinal tubuli of the fang terminate in a granular layer, which, excepting 
near the extremity of the fang, is separated from the cementum by a strongly-marked 
boundary line. 
The cells of the cementum have no peculiarity ; they are arranged in linear series, 
their long axes being in the direction of that of the tooth. The cement is tolerably 
abundant in quantity, and at the apex of the fang is pierced by canals for vessels. 
Didelphis virginiana . — The teeth of this species were from the skull of an old 
animal, and though much worn by use yet sufficient remained for my purpose. The 
marsupial characters, so far as I have had the means of observing, are strongly marked. 
The dentinal tubes on leaving the pulp-cavity have a diameter of about the oop th 
of an inch. Those that are destined for the masticating surface make one or two large 
in addition to numerous gentle secondary undulations. But all that depart for the 
sides of the crown or the tubercles, make a curve on nearing the enamel, the con- 
cavity of which is directed upwards, fig. 12 A. During this part of their course they 
give off from either side, but more especially from the convex side, numerous small, 
short, delicate branches. Finally, they break up into several branches, which on 
passing into the enamel preserve their dimensions, the 4^3^!^ RR inch. 
When within the enamel the tubes advance to near the surface, which, as the 
enamel is not thick, is no great distance. In their course, they are subject, in addi- 
tion to small undulations, to one or two deflections, the concavities of which are 
directed towards the grinding surface of the tooth, and some few give off one or two 
branches. 
The cementum is abundant, and contains numerous highly developed cells, which 
give off many branching and anastomosing canaliculi, which form a tolerably uniform 
meshwork of tubes throughout the whole mass. The extremities of the fangs are 
constituted of cement, through which anastomosing canals for vessels pass towards 
the pulp-cavity. The cementum and dentine, though separate at some parts, at 
