136 DISTINCTION INTO SPECIES. 



Mastodon diameter, of a rounded form, without the least appearance of tusks having 



Tetracau- 



lodon. ever existed. In Plate II., which is still younger than either of these, we 

 notice the tusk-sockets open and fairly displayed to view, showing the 

 great difference in the anatomical character of the last jaw from the other 

 two. On the other hand, in the great Shawangunk head (Plate XVI.), the 

 tusks having existed and fallen have left a more extended, irregular, and 

 oval surface, which, though the sockets may be said to be filled, give this 

 an aspect very different from that in the two young jaws just mentioned. 

 In regard to the rectangular outline of the jaw, it is found, on comparing a 

 number of specimens, not to be more remarkable in our Tetracaulodons than 

 in the Mastodons, so far as we can discern. Yet I must say, that an able 

 palaeontologist and anatomist maintained that there was no appearance of 

 sockets ever having existed in the Shawangunk head. 



The conclusions to be drawn from the facts above stated are the fol- 

 lowing : 1st, that the existence of tusks in the mandibular jaw does not 

 indicate a peculiar species ; 2d, that in most cases these tusks are found 

 in young animals, though one tusk is occasionally seen in the adult 

 skeleton. 



We have had an opportunity, by placing them side by side, of perfectly 

 comparing the Tetracaulodon skeleton here described with the Mastodon 

 skeleton in the collection of the University at Cambridge, which has not 

 the least appearance of a claim to the Tetracaulodon character. A com- 

 parison of the head, teeth, vertebrae, scapula, os humeri, bones of the fore 

 leg, os femoris, and bones of the hind leg, presents no difference in any 

 respect, except in magnitude. 



The pelvis in the Cambridge skeleton exhibits an aperture much larger, 

 in proportion to its width, than in the other. The whole width of the former 



