58 Geological Society. 
the socket more or less obliterated; and Mr. Owen says that the 
dimensions prove the close similarity in size and proportions between 
the lower jaws of Mastodons with and without the tusks; and 
further that no individuals of the same species could resemble each 
other more closely in the conformation of the molar teeth. In both, 
the inner boundaries of the molar series are parallel, and the inter- 
space is of the same breadth: the general form of the ascending 
ramus and the symphysis, the place and size of the great foramina 
for the dental nerves and vessels, are alike. The only differences 
consist in the Tetracaulodon* having larger condyles, and the outer 
side of the horizontal ramus being less convex and prominent ; the 
coronoid process also is higher; and the broad canal, which is im- 
pressed upon the upper part of the symphysis, is nearly straight, not 
sloping down to the deflected part as in the Mastodon; but the 
breadth of the canal is the same in both, though the symphysial part 
of the jaw is larger and broader in the Tetracaulodon than Mastodon. 
These differences, Mr. Owen observes, may relate to the additional 
motions of the lower jaw, connected with the uses to which the in- 
cisor may have been put. 
The incisor in full-grown Tetracaulodons or male Mastodons is a 
comparatively small,cylindrical and straight tusk, projecting forwards 
and alittle downwards ; its circumference is five inches; the length 
of the projecting part of the most entire of three specimens was five 
inches, but an unknown portion had been broken off ; the socket was 
three inches in depth, uniformly one and a half inch in diameter, 
and slightly concave at its termination. 
With regard to these incisor teeth and the importance attached to 
them as a generic distinction, Prof. Owen says, it must be remem- 
bered that in many species, both of Cetacea and Pachyderms, incisors 
as well as canines vary in relation to the age and sex of the same 
species of animal. In the male Dugong the upper incisors are pro- 
truded, scalpriform, and of unlimited growth, while in the female 
they are concealed, cuspidate, and solid to their base. In both sexes 
the lower jaw is provided at its deflected extremity with six incisors, 
which disappear in mature animals, only one or two remnants being 
occasionally discoverable in the cancellous sockets. In many of 
the Hog tribe, incisors are present in the young animal, but are lost 
in the full-grown. The most remarkable case, Mr. Owen says, of 
distinct conditions of incisors, teeth or tusks, relative to age and sex, 
is in the Narwhal. In this animal the young of both sexes have 
equally developed on each side of the upper jaw a single tusk, one 
of which grows rapidly in the male, constituting the well-known 
long, spirally twisted tusk, while the other remains stationary ; but 
both continue rudimental in the female. 
Were the Dugong and the Narwhal extinct, and to be judged of 
only by their fossil remains, the skulls of the two sexes of the herbi- 
vorous cetacean, viewed irrelatively, would doubtless, Mr. Owen 
observes, be referred to two distinct species, though the identity in 
* The author retains the term Tetracaulodon in his description for the 
male Mastodon. 
