Geology and Natural History. 



" The Extinct Mammalian Fauna of Dakota and Nebraska, &c.' 

 of a tooth, the original of which is lost, from a miocene formation 

 of Maryland, to be viewed as pertaining to the same species. This 

 I had named Mastodon obscurus. 



The crown of the tooth consists of four transverse divisions to- 

 gether with the merest trace of a heel. As in tlie cast of tlic 

 :\I:iryl:uid tooth, the inner lobes of the crown of tlio Ciiliforiiia 



cconis. The outer lobes, likewise as in the .Marylaiul tootli, have 

 better developed oifsets fore and aft internally than in thi' lattn-. 

 giving rise to a greater degree of obstruction of the transvt r>c 

 valleys of the crown than in the American Mastodon. Tlie fourth 

 division of the crown is proportionately less well developed, in 



the latter, but the tubercles are more equally <leveh>])e(l. Die 

 inner lobe is a single mammillary eminence not moie tliau halt'thc' 

 elevation of the outer lobe. In the Maryland tootli, tiie enrres 

 ponding lobe resembles the outer one, conssistin^ .)f a connate ]>air 

 of tubercles as well developed as in the outer lobe. Tlu- heel in 

 the California tooth, as in the Maryland tootli is tunned l)y a siiort 

 mammillary eminence occupying the angular space posteriorly of 

 the lobes of the fourth division of the crown. A l)asal ridge is 

 better developed externally in the California than in the Maryland 

 tooth. 



Comparative measurements of the California tooth, with the 

 cast of the Maryland tooth, and one of the Mastodon Americanos 

 are as follows : 



Depth of do. internally, '' 2 in! 2 lines. 2 in. 2 lines. 3 in. line-. 



It is not improbable that the California tooth may have per- 

 tained to the same species as the fragment of tusk previously 

 noticed, and, perhaps these, together with the Maryland tooth, 

 and others previously referred to Mastodon obscurus, may likewise 

 belong to the same animal. The positive determination of this 

 question must be left for the discovery of additional material to 

 throw light on the relationship of the different specimens which 

 have been thus far presented to our notice. 



2. On the cause of the Motion of Glaciers ; by J. Ceoll, of the 

 Geol. Survey of Scotland, (Phil. Mag., Sept., 1870).— Mr. CroU 

 closes his article on the cause of the motion of glaciers with the 

 following— ow the present state of the question, and on the alleged 

 limit to the thickness of a glacier. 



* Partially estimated, as the specimen is imperfect at its fore part 



