1894 - 95 .] Dr J. Smith on the Mammalian Tooth. 
337 
were slightly rotated on its long axis, to the development of what 
resembles a complete screw thread winding round its whole length 
as a central pillar or columella, the thread of such screw increasing 
in breadth and thickness with the diameter of the conical surface 
round which it is developed. 
The Monodon or Narwhal (fig. 5) affords, perhaps, the best marked 
example of a spiral tooth with which naturalists are acquainted. 
In the tusk of this animal the whorls forming the spire are very 
complete throughout its whole length, which sometimes extends to 
10 or 12 feet. In perfect specimens each whorl is separated from 
the other by a well-marked suture, forming a screw of considerable 
pitch, and winding round the tooth in such a manner as to render 
its appearance similar to that of some of the tapering spiral shells 
of the Gasteropods, such as the Turritelidse. These teeth in the 
Narwhal have been erroneously described, even by such authorities 
as Cuvier, Owen, C. S. Tomes, and others, as incisors. Professor 
Sir William Turner, however, has shown them to be unmistakably 
canines, and belonging entirely to the true maxillary bone of each 
side. In their case a curious departure takes place from that sym- 
metry of form normally exhibited in corresponding structures on 
the two sides of the body. Of these two canines there is rarely 
more than one developed, and that is the Left one, the Right being 
usually suppressed. In some instances both teeth are present, but 
there appears to be no instance of the Right tooth being developed, 
and the Left suppressed. In this left tooth the spiral winds from 
right to left, constituting what is familiarly known as a left-hand 
screw ; occasional examples, however, have been met with where, 
in the adult animal, both teeth are developed, and what seems 
remarkable in these cases is, that the direction of the screw — that 
is, from right to left — is found to be the same in each tooth ; while 
this arrangement is still more widely established as the general rule, 
by finding in the foetal Narwhal, of which more numerous specimens 
are at our command, that the spiral markings, although less distinct, 
are of the same character in the two teeth, while neither one nor 
other of them have been as yet erupted. Curiously enough, the 
direction of the screw characteristic of these teeth corresponds to 
the principle of construction prevailing in the mammalian dentition. 
Another group of the cetacea, known as the Ziphiods, affords an 
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