80 
COMPARATIVE ANATOMY OF TEETH. 
since their teeth are so much alike—not being divided 
into incisors, canines, and molars. They are mostly 
pointed and sharp, like canines; but you may fall back 
on structure to some extent. The bulk of the teeth of 
most fishes is made up of one or other modification of 
vasodentine or osteodentine; this is often glazed over upon 
its exterior by a thin film of enamel, so thin as often to 
appear structureless. Unvascular dentine also forms the 
teeth of many fishes, and in some is remarkable for the fine¬ 
ness of its tubes; in fact, every form of dentine, from fine 
tubed hard dentine to tissue undistinguishable from coarse 
bone, is to be found in this class. The formation of the 
condyle of the lower jaw and the shape of the glenoid fossa 
of the temporal bone is of great assistance, as corroborating a 
diagnosis of the tooth. This will show whether the animal 
was a ruminant or a flesli-eater by the degree of motion 
permitted to the lower jaw. 
Then, again, as to number of teeth. There are homologies 
in number, and it will much assist the naturalist in the 
identification of an animal through its tooth to know which 
of a number of teeth any particular one is ; as for instance, 
whether it be the first, second, or third incisor, the first, 
second, third, or fourth premolar, or which molar, just as a 
dentist should be able to name the exact position in the jaw 
once occupied by any human tooth that you might place before 
him. The typical number of teeth in mammalia is believed to 
be:—Incisors, 8-8; canines, 1-1; premolars, 4-4; molars, 8-8— 
equal to 22 in each jaw, or altogether to 44. The number of 
teeth (82) in man is interesting, as being not very far from 
that which is typical of the great bulk of the class to which 
he belongs. It is identical with that existing in the whole 
of the apes which inhabit the old world, and those of the new 
world only differ from him by the presence of one more pre¬ 
molar, or by the absence of a molar on each side of each jaw. 
In man’s own class (the mammalia) the number of teeth 
developed may be very great, as in the dolphins, where the 
greatest number is reached in Pontoporia, namely, 220; 
while in the narwhal the teeth are reduced to two, only one 
of which is fully developed. Passing out of man’s class, teeth 
in the reptiles may be many or few; but amongst fishes we 
meet with every extreme, from a single-pointed tooth on the 
roof of the mouth, as in myxine, or two above and two below 
(flat and crushing), as in Ceratodus , up to such a multitude 
that to count them would be a task both useless and difficult, 
as in muraena—a sort of eel—and the common pike. 
The development of teeth next demands our consideration. 
They are perfectly distinct from the internal bony skeleton, 
