74 Annals of the Carnegie Museum. 



which unites with the rather well-developed parastyle, while in the 

 posterior molars the cingulum is rather poorly developed, as is also 

 the metastyle. There is no mesostyle, but a well-developed ridge 

 near the outer face of the crown, which effects a connection between 

 para- and metacones, unlike what is seen in Bunomeryx or Hylomeryx. 

 Internally at the exit of the median valley there is (especially on M-) 

 a heavy, smooth cingulum, which is most suggestive of similar cingula 

 on the molars of Helohyus and of such later Tertiary selenodont forms 

 as Dromomeryx, Palceomeryx, and Dicrocerus. P- has the proto- 

 and deuterocone of equal size and a heavy cingulum anteriorly and 

 posteriorly, which terminate externally in small basal tubercles. 

 P- has a large protocone with a quite trenchant ridge extending back- 

 ward and sloping gradually. The tooth is surrounded by a cingulum 

 especially well-developed internally, so that one may say there is a 

 rudimentary deuterocone. This deuterocone or tubercle is located 

 further forward on the tooth than in Bunomeryx or Hylomeryx. 

 P- is represented by two roots. 



Measurements. 



to M^ i6 mm. 



4-5 



3-7 



5 



3-5 



- 4-5 



- 5 



" 6 



• 5 



Phylogeny. 



There is but little doubt that Mesomeryx belongs to the subfamily 

 HomacodontincB. From the characters of the molars one would not 

 long hesitate in placing the genus near Sphenomeryx or Bunomeryx. 

 The consolidation of the antero-internal and antero-median tubercles 

 into a cross-ridge as in Hylomeryx, and the absence of the hypocone 

 on M- as in Sphenomeryx, may in this genus be looked upon as repre- 

 senting a line, which paralleled the evolutionary stages in the Homa- 

 codon-Sphenomeryx phylum. 



It_is quite evident that the foregoing genera are most nearly related 

 to the Bridger genus Ho^nacodon. That these upper Eocene Artio- 

 dactyls of America hold a position relatively well differentiated from 



