Professor Owen’s Report on the Missourium. 57 
todon, like that of the Elephant, being inserted by a nearly straight 
cylindrical base in a socket of corresponding form, and can be 
rotated in any given direction when the natural attachments are de- 
stroyed by decomposition ; and he alludes to the skeleton exhibited 
in London in 1805, in which the tusks were bent downwards. 
Having, by a series of comparisons of the teeth and bones, which 
the author does not conceive it necessary to recount, arrived at the 
conclusion that the Missourium is either a Tetracaulodon or [a] Ma- 
stodon, he next considers the relations in which these supposed di- 
stinct genera stood to each other; premising that Mr. Koch’s ske- 
leton illustrates the osteology of the gigantic Mastodon far more 
completely than has been done by any other collection of North 
American fossils brought to Europe. The genus Tetracaulodon was 
founded by Dr. Godman on the lower jaw of a young Proboscidean 
having 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 have 
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. 
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 
