Geological Society. 149 



jaws of full-grown animals ; three which retained the right tusk or 

 exhibited its socket, and three in which the tusk was wanting, and 

 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 a little 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 



* The author retains the term Tetracaulodon in his description for the 

 male Mastodon. 



