ANATOMICAL PECULIARITIES OF TEETH. 209 
ditions, makes up a large proportion of the tooth in 
some whales. This identity of tissue, arising in con- 
sequence of disease, in one animal with that which is 
normal in another, called forth the remark from Goodsir, 
that it “must be looked upon as another instance indi- 
cating the existence of a system of laws regulating the 
relations between healthy and morbid tissues.” 
In this connection a short reference to a curious 
affection of the teeth of whales may be made. In the 
erampus and sperm-whale the teeth consist of a hollow 
cone of dentine coated by cementum; the pulp chamber 
is very large and contains a quantity of osteo-dentine. 
In one species, Orca, Eschricht found a complete covering 
of enamel to the free portion of the tooth. In Orca, 
/as in the porpoise, the teeth interdigitate when the 
mouth is closed, and as the teeth wear, owing to the 
friction, their broad bases come more into apposi- 
tion. This wearing of the contact surfaces must, in 
the long run, inevitably open up the pulp chamber; 
as a rule it is prevented in consequence of the formation 
of osteo-dentine by the pulp. In some cases the de- 
velopment of osteo-dentine does not take place with 
sufficient rapidity, and the pulp becomes exposed and 
subsequently destroyed. A case of this nature has been 
minutely described by Eschricht and by Tomes. 
No order of mammals exhibits so many deviations 
from the ordinary conditions of teeth as whales: not 
the least interesting abnormality in connection with our 
present purpose is the remarkable overgrowth of the 
teeth of AZesoplodon, secured by the naturalists of the 
Challenger Expedition, and carefully described by Pro- 
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