VERTEBRATE FAUNA OF THE BLANCO BEDS. 65 



On the supposilion that one cross-crest has been lost, the crown con- 

 sists of five cross-crests, or four cross-crests and a very large heel, which 

 has the constitution of tlie cross-crests, but is transversely contracted. 

 The cross-crests are remarkable for their acute summits, in which they 

 resemble those of M. ammcanus and T. serridens. They are, however, 

 more contracted transversely, or shorter at the apex than is the case with 

 either of those species. In their acuteness they differ from both the spe- 

 cies which are found in the same formation. They have accessory ridges 

 on the external halves only, which would produce lobes on wear, while 

 the internal halves of the crests are entirely simple. These ridges have 

 no distinct apices, but disappear upwards in the cross-crests, from which 

 they are separated Ijelow by shallow open grooves only. They are want- 

 ing at the posterior bases of the third and fourth crests. They do not 

 reach the median line, but the main axis is produced beyond them to 

 reach it. The section of an external crest is then rather a four-lobed 

 cross than a trefoil. The apices of both halves of the crest descend rap- 

 idly to the median fissure, meeting at a strong angle a little more than 

 right. The edge of the internal half of the crest is slightly notched or 

 bilobate, except on the fifth crest. The valleys are bounded on both 

 sides of the crown by tubercular ledges, but there are no lateral or pos- 

 terior cingula. Enamel smooth. A considerable deposit of cementum 

 between the crests. 



Measurements. 



Mm. 



Length of last molar (anterior crest restored) 160 



Width same at second crest 68 



Width same at fifth crest 41 



Elevation of second crest 54 



Elevation of fifth crest 30 



Width of apex of second crest 29 



Width of apex of fifth crest • 18 



The tusks of this species are unknown, and I refer it to the genus 

 Dibelodon provisionally only. It must be compared with the T. sliepardii 

 and the Mastodon americanus, as the species to which it bears the nearest 

 resemblance. It differs from the former in the five cross-crests, which are 

 more elevated, more contracted in both directions and more acute. The 

 outline of its base is more wedge-shaped. It differs from the M. americanus 

 in the same features, and in the presence of accessory ridges of the external 

 half of the cross-crests. Of all the Mastodons of America it approaches 

 nearest to the M. americanus in the simplicity and acuteness of the 

 crests, and further information as to its characters will be welcome, in 

 view of a possible relationship with the latter species. 



5-Geol 



