1894 - 95 .] Dr J. Smith 07i the Mammalimi Tooth, 343 
axial rotation, as well as in the external form of the tooth. The in- 
cisors of the Horse afford another obvious and well-defined example 
of the same peculiarities (figs. 11 and 12). These teeth, crown and 
fang conjointly, are bent backwards so as to form an arc having its 
convexity directed anteriorly. Seen from the front, the tooth appears 
as a cone of which the cutting edge represents the base and the 
apex of the fang the vertex, such cone being twisted on its long 
axis in a similar manner to the other teeth already noticed. Upon 
closer inspection, however, the appearance of the fang seems 
to indicate something more than mere axial rotation, and rather 
suggests the winding round it of what would be equivalent to a 
double screw thread. Taking an upper incisor as an example, one 
such thread is traceable in a somewhat salient track winding very 
obliquely round the front of the tooth from the distal side of the 
crown towards the mesial side of the fang ; and, posteriorly, another 
similar thread is found, now, however, winding from the mesial 
side of the crown to the distal side of the fang : thus constituting 
a double-threaded screw combination developed on the conical body 
of the root. In using the term “ screw thread,” it must, of course, 
be understood as employed not with geometrical exactitude, but 
merely as indicative of an oblique form of surface development 
imparting to what would otherwise be merely a simple cone or wedge 
a certain amount of a halicoid conformation, probably giving addi- 
tional steadiness to the fang, and altering the nature and direction 
of those forces acting upon a tooth in the exercise of its functions. 
Among the Primates, the large canines of the male Gorilla, the 
Orang, and the Mandrill, may serve to exemplify how the teeth in 
these animals accord with the description of the turn apparent in 
the crown and fang of others already mentioned. But, as we 
ascend in the scale of animal life, these characters seem to become 
less perceptibly marked until the human species is reached, when 
little more than vanishing traces of them seem to remain. 
This comparatively feeble marking of the teeth in Man makes it 
more difficult to recognise in them so perspicuously the principle of 
formation found in the more typical illustrations adduced. And, in 
addition to this fading from the original model, the disadvantage 
in the human tooth is increased by malformations and irregularities 
in the form and development of the fang — there being like 
