THE MAMMALIA OF THE DEEP RIVER BEDS. 



153 



ancient form the premaxillaries are very broad, very much depressed and flattened, 

 and the broad surface presents upward ; the anterior nares are rather lower, but the 

 narial notch is deeply incised, extending back over pJJ. In M. montanus, on the 

 other hand, the premaxillaries present their broad surfaces anteriorly, not superiorly, 

 which makes the alveolar less depressed and of greater vertical depth ; the narial 

 opening is higher, but does not extend so far back, only a little behind the line of 

 the canine. 



The mandible also differs much in the two species. In M. montanus the horizon- 

 tal ramus is somewhat deeper and has not so straight an inferior border ; the sym- 

 physis is longer, straighter and more inclined, less procumbent and projecting at the 

 incisive alveolus. The angle extends to a much less degree and less abruptly below 

 the inferior border of the horizontal ramus, but, on the other hand, it projects farther 

 behind the plane of the condyle, from which it is separated by a more decided notch. 

 The masseteric fossa is smaller, but deeper and more distinctly demarcated. The 

 coronoid is very peculiar; it is short, broad and blunt at the tip, and projects forward 

 much more than upward, in consequence of which the coronoid notch is very broad 

 and shallow. 



The dentition has no very marked peculiarities. In the upper jaw the incisors 

 are very small, with compressed, simple and obtusely pointed crowns ; they increase 

 regularly in size from the first to the third. The canines are very large and of the 

 trihedral shape usual in the family. The anterior premolars have crowns which are 

 low bat long antero-posteriorly, with trenchant margins ; M is less extended in the 

 fore-and-aft direction. The molars increase in size from the first to the third and 

 exhibit little peculiarity of structure. The external buttresses or styles are quite 

 prominent, but the metastyle of mjs, the enlargement of which is so constant a 

 feature in species of Merycochcerus as to be of generic value, is smaller than in the 

 John Day species. The para- and mesostyles are prominent, but thin and com- 

 pressed, and the external faces of the para- and metacones are but slightly concave. 



In the lower jaw the incisors are considerably larger than the corresponding 

 upper series and have high, chisel-shaped crowns ; that of the median incisor is very 

 narrow; the second is broader and the third still more so. The canine is functionally 

 one of the incisors, but is much larger than any of those teeth and its crown is 

 pointed rather than chisel-shaped, with obliquely descending supero-external border, 

 which is compressed and trenchant. 



As in the oreodonts generally, pTi has assumed the form and function of the 

 canine and is very robust. The crown of pTa is compressed, trenchant and elongate 

 antero-posteriorly ; the deuteroconid is represented by a ridge on the inner face. p. a 



