338 Proceedings of Uoyal Bocidy of Edinburgh. [sess. 
almost equally well-marked although somewhat different form of 
spiral construction in the teeth of some of its species, as in the 
Mesoplodon Layardi described by Professor Turner in the “ Chal- 
lenger Reports.” This animal presents some very remarkable 
features in the character of these organs : two dental structures 
exist on the lower jaw, one on each side, which curve upwards and 
outwards, then obliquely backwards, and finally turn inwards until 
they cross each other like a loose belt buckled above the upper jaw 
or snout, so that the lower jaw cannot be depressed, or the mouth 
opened, except to a ver}^ limited extent. The curve or spiral taken 
by these teeth is more difficult to describe, or even to recognise, 
than in the case of the Narwhal ; but, unlike it, the spiral is sym- 
metrical — that is, it takes a reverse direction on opposite sides of 
the head. By far the largest portion of the tooth is formed by the 
fang. This consists of a flat broad plate, somewhat crescentic or 
falciform in its outline, with the concave edge looking upwards and 
forwards — one extremity of the crescent being rooted in the lower 
jaw — from which this flat crescent-shaped plate rises with its inner 
surface in relation to the outside of the upper jaw, and is finally 
carried backwards, and bent or twisted across the dorsum of the head 
or snout (fig. 6). It is exclusively in the fang of the tooth that this 
peculiar contour is established, the crown proper taking no part in 
such flexure, since that portion of the tooth is represented merely 
by a small triangular protuberance situated upon the flat surface of 
the upper and extreme end of the fang. The somewhat complex 
curvature here described approaches what would result from wind- 
ing a flat strip of flexible metal or other material very obliquely 
and for about one turn round a column of considerable diameter — 
such columnar axis being then withdrawn, leaving the strip to 
represent the thread of the screw thus formed. 
The two examples here given may serve to typify the modes in 
which a spiral construction presents itself in the long axis of certain 
mammalian teeth — being in the one case illustrated by the thread 
of the screw-nail, and in the other more or less by the worm of 
the cork-screw. Along with such spiral configuration, however, 
which seems almost universal, it is impossible to overlook another 
peculiarity of form in many teeth, and with which it is indissolubly 
associated. This is the bend or arching throughout their whole 
