240 ON THE EXTINCT MAMMALIA OF 



MASTODON. 



Remains of the Mastodon which have come under the inspection of the author 

 apparently indicate four distinct species as having once inhabited North America. 

 One of the species, the well known Mastodon ohioiicus, or M. americanus as I shall 

 hereafter call it, appears to have roamed throughout the continent during the quater- 

 nary period. A second species, perhaps the Mastodon andium, is indicated by a molar 

 tooth, obtained at Tambla, Honduras, by Capt. J. M. Dow, and presented by him to 

 the Academy. A third species seems to be represented by a few remains purporting 

 to have been derived from the miocene formations on the Atlantic border of the 

 United States. The remains of a fourth species. Mastodon mirificus, were discovered 

 by Dr. Hayden in the pliocene fossiliferous sands of the Niobrara River, in associa- 

 tion with a multitude of other fossils described or mentioned in this work. 



Mastodon americanus. — In this species the molar series consists of six teeth to each 

 side of both jaws. They successively increase in size from first to last. The anterior 

 three widen posteriorly, the succeeding pair are of nearly uniform breadth, and the 

 last one narrows posteriorly. 



The crown of the first tooth presents two principal transverse divisions or ridges ; 

 that of the second tooth two principal ridges and a third rudimental one ; the crowns 

 of the succeeding three teeth each three ridges ; and the crown of the last tooth four 

 principal ridges and a fifth elementary one more or less reduced to the condition of 

 a heel. 



The divisions or ridges of the crown form wide hills, with sub-acute summits, sepa- 

 rated by transverse angular valleys. The latter are slightly shallower at the middle 

 of their course, but are not interrupted by accessory eminences as in the European 

 Mastodon angustidens and some other species. 



Constituent elements of a basal ridge, more or less well-developed, occupy the 

 bottom of the crown, usually in front and behind and at the outlets of the transverse 

 valleys, and moi-e rarely at the bottoms of the ridges internally and externally. 



The transverse ridges of the crown are sub-divided by a median wide notch, 

 narrowing to a vertical cleft, into a pair of pyramidal lobes, which are connate from 

 the base for three-fourths of their length. The sub-acute summits are prolonged 

 towards each other, and occasionally are somewhat denticulated. The exterior sides 

 of the lobes are convex and sloping. The anterior and posterior surfaces are broad 

 and sloping planes, often slightly convex or concave, and they form the sides of the 

 transverse valleys. They ai'e nearly even, but often present longitudinal rugose 

 ridges separated by irregular grooves, especially towards the bottoms of the valley's. 



The outer lobes of the iufei'ior molars, and the inner ones of the superior molars, 



