690 
naving two tusks projecting from the symphysial extremities. Mr. 
W. Cooper of New York, however, suggested that the Tetracaulo- 
don was nothing but the young of the gigantic Mastodon, and that 
the tusks were lost as the animal became adult. This opinion has 
been also advanced by others, but without being illustrated by any 
analogies; and it has been opposed by Dr. Isaac Hays, in an elabo- 
rate memoir on additional specimens, which he states present all 
the proofs necessary for refuting the opinion that Dr. Godman had 
committed the error of describing as a new animal the young of a 
known species; and he observes with respect to Mr. Titian R. 
Peale’s suggestion that the lower tusks might be only a sexual di- 
stinction, “‘ that it is impossible in the existing state of our know- 
ledge, and with our present materials, to confirm or positively refute 
this suggestion.”’ The most recent opinion on the subject, Mr. Owen 
states, is contained in the last edition of the ‘ Ossemens Fossiles,’ in 
which M. Laurillard, after alluding to the opinion that the lower jaws 
with tusks may be immature Mastodons, proceeds to say, ‘‘ others 
have been led to believe that the lower jaws of every age which haye 
tusks belong to a different species of large Mastodon: some charac- 
ters taken from the form of the jaw would seem to justify that opi- 
nion.”’—Oss. Foss. 8vo. vol. ii. p. 373, 1836. 
Mr. Koch’s collection of detached bones contains, Mr. Owen 
states, a number of lower jaws with the molars of Mastodon gigan- 
teum, which prove the important fact, that an animal of the same 
size and molar dentition as the Mastodon was characterized in the 
adult state by a single tusk projecting from the symphysial extremity 
of the right ramus, and that the two inferior tusks are manifested 
only by immature animals. 
Mr. Owen then details the evidence by which he arrived at the 
conclusion that the Tetracaulodon of Dr. Godman is the immature 
state of both sexes of the Mastodon giganteum, that in the adult male 
only one of the lower tusks is preserved, and that in the adult female 
both are wanting. 4 
A table is given in the memoir of the measurements of six lower 
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 beg less convex and prominent; the 
# The author retains the term Tetracaulodon in bis description for the 
male Mastodon, 
