340 Proceedings of Royal Society of Edinburgh. [sess. 
however — making allowance for such flexure— is still quite trace- 
able in all these examples, and is directed, as it generally seems to 
be throughout the mammalia, from the mesial side of the base 
towards the outer side of the free extremity of the tooth, or as if the 
exposed portion of the tooth had been rotated slightly backwards 
and inwards. In the Hippopotamus this configuration, owing to the 
trihedral form of the canines, admits of a different mode of demon- 
stration (fig. 8). The lower canine — the most typical tooth in this 
animal — presents three surfaces, constituting a prism, with one of its 
faces directed posteriorly. On making a more careful examination 
of this face, it will he found that at the base of the tusk it looks 
partly outwards as well as backwards, while, as we approach its 
apex, the aspect of this posterior plane is gradually altered till it 
looks exclusively backwards, thus showing the tooth to have been, 
as it were, rotated backwards and inw'ards at its extremity for 
about a quarter of a turn on its long axis. 
In the Suidae, again, both the axial rotation and the bending of 
the tooth upon itself are well represented. Taking the canines of 
the Phacocaerus or Wart-Hog as a typical example, they constitute 
horn-like organs projecting from the sides of the mouth, those from 
the lower jaw curving outwards and upwards, while the formidable 
tusks of the upper jaw curve outwards, — forwards and then upwards 
also, — so that when the mouth is closed the anterior convexity of 
the upper is in relation with the posterior concavity of the lower 
tooth (fig. 9). Greater complexity in regard to the direction of 
the long axis spiral is here occasioned not only by this remarkable 
corniform bend or arching of the tusk, but by the upper tusk 
pointing upwards while properly it ought to point downwards 
instead. While this upward curvature of the upper canine is 
unusually conspicuous in the Wart-Hog, the same peculiarity attains 
its maximum character in the Babyroussa Hog, where the upper 
canines assume this upward bend, not after issuing from the mouth, 
hut where they seem actually to emerge from the upper aspect of 
the superior maxilla, piercing the integuments above them, and pro- 
truding through the skin at each side of the snout (fig. 10). In this 
manner they present the appearance of two long and slender horns 
arching upwards and backwards, each describing more than a semi- 
circle, the extremity of which just clears the side of the animal’s 
