84 
COMPARATIVE ANATOMY OF TEETH. 
amount of grain and even young trees, you have first a 
layer of cementum, then a layer of very hard enamel which 
will turn the edge of the hardest steel instrument, and soon 
spoil a file, and then a layer of dentine. So that, as will be 
seen from the transverse section of an elephant’s tooth, it is 
quite impossible to polish it evenly, and it cannot but he felt 
that the surface is rough. You can distinguish the African 
from the Asiatic elephant by its tooth. In the African the 
enamel winds in and out in two lines like the sinuous course 
of the sea serpent. In the Indian the enamel forms rings or 
long oval islands in the tooth. 
I have already alluded to the fact of the elephant’s tusk 
growing from a persistent pulp, as does that of the narwhal 
(Monodon monoceros ), the ancient unicorn, whose tusk will 
grow to a length of ten or twelve feet; but we have interesting 
examples of persistent pulp and continuous tooth-growing 
nearer home, in the rodents—the rats, rabbits, hares, etc. 
Here I have to introduce to your notice one of the most 
beautiful specimens I have seen, specially lent to me to show 
you by my friend, Mr. F. H. Balkwill, L.D.S., of Plymouth. 
It is the skull of a rat. The lower incisors have by some 
means become inclined to the left, and missing the upper 
ones, have not been worn away by them, and have grown 
upwards, curling backwards an inch long; while the right 
upper incisor has grown in a circular and spiral direction, 
completing a circle and a half and projecting from the side of 
the palate bone about tliree-eigliths of an inch. The left 
incisor has likewise curled round, but has penetrated the 
margin of the pre-maxilla, and its point is shown by a small 
portion of the bone having been cut away about one-eiglith 
of an inch short of completing the circle. These gnawing 
animals would soon be without teeth did not their incisors 
grow as fast as they wear them down. There are many 
examples in museums of an incisor tooth which, from some 
irregularity of position or from having nothing to oppose it, 
has grown and grown in a circle until the point of the tooth, 
recurving on the head, has either pierced the skull or so 
prevented the animal opening its mouth that it has died of 
starvation. At our last Goose Fair, in the wild beast show, 
the keeper showed a large handful of chewed wood which was 
made by the porcupine, to whom they were obliged to give a 
chump of wood every day upon which he might exercise his 
teeth. There are many points of great interest to naturalists 
in the teeth of snakes, insect-eating animals, and the 
carnivora ; but one would have to write a book and give very 
many illustrations to do more than touch the borderland of 
this extensive subject. 
