148 Geological Society. 



tusks in the skeleton exhibited at the Egyptian Hall, Mr. Owen 

 states, that it may have arisen from compression, the tusk of the Mas- 

 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 j 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 



