THE MAMMALIA OF TIIK UIXTA FORMATION. 509 



The first preniohir is represented merely by the fang, which is riidiinenlary 

 on one side. The second preniohir has a slight swelling on the outer face which 

 anticipates the posteroexternal lobe; there is a single prominent internal cusp united 

 by two low subcqual ridges witli the external crest; surrounding the inner lobe is 

 the prominent cingulum which also characterizes all the succeeding molars. The 

 third and fourth premolars are interesting as showing a mode of development of the 

 2)OSterioi- ci'ests quite unlike that in the tapir or Equus series ; there is as yet no redupli- 

 cation of the internal lobe as in the latter series, but the posterior crest grows directly 

 out from the bilobate external crest in the valley formed by the prominent cingulum 

 and the much broader and more elevated anterior crest ; these two teeth are practi- 

 cally similar and submolariforra, except that the third premolar has a less distinct 

 posterior lobe and crest. 



The first true molar is very similar to that in Aceralheriuin ; the anterior crest 

 has a slightly marked "anticrochet'" which is entirely wanting in ni.- and m.'; the 

 external crest is perfectly flat and decidedly inclined inwards. The second molar has 

 the same characters with a very decided extension of the external crest beyond the 

 posterior transverse crest. In the third molar the external crest arches out widely 

 beyond the posterior crest; the base of the crown is subquadrate in outline. 



ilensurements. 



A. antiquus. A. inUrmedius. Metamynodon planifrons. 



M. M. M. 



Antero-postcrior (liami'ter of nicdiiin lower incisor OH (ii|)per do. ) .012 



Fore and aft (linmcter of crown of lower canine al base. .015 (upptrdo.) .031 .035 

 Upper jaw. 



Fore and aft diameter of third premolnr 01!) .021 



Length ot entire molar series .187 .225 



" true molar 101 100 



Fore and aft diameter first molar 037 .044 .017 



Transverse " " " 0:n .043 .0(W 



Fore and aft diameter, second molar 04."i .058 



Transverse " " " 037 .052 



Fore and aft diomcler. third molar 028 .046 .OfiO 



Transverse " " " 030 .040 .Olil 



Measurement of canines, outside .115 .130 



The sluU. The most striking features of the skull are at once seen to l)e the 

 great breadth of the ni .i x i I lari es opposite the canines and their deep excavation 

 between the canines and the antorbital ridge; the snout is thus broader than the 



A. r. s. — VOL. xvr. 3.m. 



