THE MAMMALIA OF THE DEEP RIVER BEDS. 85 



long, low ridge, which is not connected by a transverse crest with the outer wall of 

 the crown. The tetartocone is low and conical in shape ; on wear it becomes confluent 

 with the tritocone. Strange to say, this tooth is proportionately less reduced thau 

 in the "White River equine, Mesohippus. The second premolar is the largest tooth 

 in the series and differs in several details of construction from all the others ; the 

 enlargement affects particularly the anterior portion of the crown. The protocone 

 (antero-external cusp) is elongate in the antero-posterior direction ; it is but very 

 slightly concave and the median rib is prominent. The protostyle, or anterior but- 

 tress, is greatly enlarged, and to it much of the characteristic appearance of the 

 tooth is due. The tritocone is much shorter (from before backward) than the proto- 

 cone, is more concave on the external side, and the median rib is nearly obsolete. 

 The deuterocone is fused with the anterior conule, though a constriction indicates 

 clearly the limits between them, and the thickness of the conule renders it a very 

 conspicuous element of the crown ; the anterior transverse crest is approximated to, 

 but does not coalesce with, the outer wall. The tetartocone is somewhat less dis- 

 tinctly constricted from the large and prominent posterior conule, and this makes the 

 posterior transverse crest very much broader than in Ancliitherium ; it is confluent 

 with the outer wall, and the enamel on its anterior margin is slightly crenulate. The 

 posterior pillar is large and of triangular shape ; in the stage of wear exhibited by 

 the specimen, the pillar has become confluent with the tetartocone, though the limits 

 of the two cusps are marked by a fold in the enamel covering. A small pillar or 

 style arises also from the anterior side of the tetartocone, but doubtless this, as in 

 AncTiitherium, is an individual character. Although this tooth is typically brachyo- 

 dont, cement is deposited in the anterior and posterior valleys, but apparently not in 

 the median valley or the outer or inner sides of the crown. 



The third and fourth premolars differ from the second chiefly in matters of detail. 

 The proto- and tritocones are of more nearly equal size and shape, though the former 

 still exceeds the latter in size ; it is somewhat concave on the external face and the 

 median rib is obsolete. The posterior crest is angulate and sends forward a process 

 which nearly reaches the deuterocone ; its front margin, especially on M, is much 

 more markedly crenulate than in M. 



The molars decrease in size from the first to the third ; their construction closely 

 agrees with that of the premolars, but the external cusps (para- and metacones) are 

 of nearly equal size and less concave than the corresponding elements of the premo- 

 lars. The conules, especially the anterior, are somewhat more separated from the 

 inner cusps. The posterior transverse crest is confluent with the outer wall, but the 

 anterior is not. 



