Species of the Secondary and Tertiary Formations ? 251 



The discovery of such an animal, which has no analogy to 

 the mammifera actually living on the continent of New Hol- 

 land, has led Mr Owen to suppose, that this Mastodon might 

 belong to an entirely different family. He has, therefore, 

 referred this great humatile quadruped to a group of Marsu- 

 pials or Didelphous Mammifera, whose principal types repre- 

 sent, in this continent, the parallel types of the different 

 families of the great classes of mammifera. 



Mr Owen has not admitted this supposition without doubt;* 

 but it does not the less prove how difficult it is to be certain, 

 not only of the exact determination of a race belonging to 

 geological times, but even of the family or class to which it 

 belonged. 



These difficulties are so much greater, because it is far 

 from being settled what we ought to understand by a species, 

 especially when we wish to verify experimentally, whether 

 generation confirm, or otherwise, the distinctions which have 

 led us to consider them as different, or the analogies by which 

 we seek to assimilate them. Do the marine species undergo 

 gradual modifications by a change in the degree of saltness 

 of the waters in which they live, or in consequence of any 

 other exterior circumstance ? Who is unacquainted with the 

 effects these causes produce on the mollusca, and, for ex- 

 ample, on many species oiMurex, Pterocera, Buccinum, Oliva^ 

 and particularly on the Cardium edule f 



Thus, according to Mr Gray, the shells of Buccinum un- 

 datum, and of Buccinum striatum of Pennant, differ only in 

 this, that the former, formed in agitated waters, becomes 

 thick and heavy. The second, which has lived in the tran- 

 quil waters of harbours, has there become light, smooth, and 

 often coloured. 



The shells which are provided with branched or dilated 

 prominences, such as Murez, are likewise subject to great 

 changes, according to the circumstances in which they happen 

 to be placed. Many varieties, produced by local causes, have 

 been improperly considered as distinct species. 



* New researches, and more complete bones, have shewn, that in reality this 

 animal was not a Mastodon, and that it ought to occupy a place in the sub-class 

 of Marsupials. 



