MASTODON TETRACAULODON. 135 



fourth in this Tetracaulodon, and seven and a half in this Mastodon ; thus Mastodon 



Tetracau- 



showing that, in this instance at least, the part is not longer in the Tetra- i don. 

 caulodon, but shorter than in the Mastodon G-iganteus. In the same jaws 

 there is no difference in the declination of the condyloid process, and the 

 semilunar notch is not deeper in the latter than in the former. The trun- 

 cated appearance of the anterior or rostrated portion is, as Dr. Goclman 

 states, more frequently met with in the Mastodon Giganteus, but is some- 

 times seen in the Tetracaulodon. 



This truncated portion deserves some particular notice, as it is the seat 

 of the Tetracaulodon tusks, where they exist, and its appearance indicates 

 whether they have existed or not. In the great Mastodon head, this rough 

 surface, being of a somewhat circular form, is, as we have seen, about three 

 inches in diameter. In jaws in which both tusks exist, or from which they 

 have recently fallen, this •surface presents an oval form; the circle being 

 flattened from above downwards, and divided into two parts by the ridge 

 which separates the tusk-cavities. If the tusks originally existed, and 

 have been shed for a length of time, the cavity which contained them is 

 more or less filled with osseous matter, which sometimes renders it difficult 

 to decide whether they have existed or not ; but, in the greater number of 

 cases, a sufficient depth of cavity remains without osseous matter, to show 

 that sockets have originally existed ; and this fact is supported by another, 

 viz. the transversely oval form it exhibits. Where there is, in this truncated 

 extremity, nothing to lead one to suspect the former existence of tusks, it 

 is of a rounded form ; being, in the Baltimore jaw, about three inches in 

 diameter. 



In two young jaws belonging to the Cambridge collection, and repre- 

 sented in Plates III. and V., the truncated extremity is two inches in 



