22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 128 



The lower molars may be a little more hypsodont and possibly 

 more crescentic than in Bunomeryx. In the lower premolar series, 

 the paraconid of P3 in Pentacemylus is more clearly defined and that 

 in P4 is distinctly larger. 



Pentacemylus very likely evolved from Bunomeryx, and the differ- 

 ences between the two are of no great magnitude. These dififerences, 

 however, appear to be definitive and rather significant considering 

 the shortness of the interval of time separating the known materials 

 of the species of each. 



PENTACEMYLUS PROGRESSUS 10 Peterson, 1931 

 Plates 2 (above) and 3, figures 2, 3 



Type. — Upper molars M^ and M^ and lower teeth, including Mi, 

 M2, and part of P4, CM. No. 11865. 



Horizon and locality. — Randlett horizon of Duchesne River forma- 

 tion (upper Uintan), 3 miles north of Leota Ranch and i mile west 

 of Green River, Uinta County, Utah. 



Discussion. — The species Pentacemylus progressiis is much larger 

 than Bunomeryx montanus and only a little larger than Hylomeryx 

 annectens. P. progressus and the new form herein described as 

 Mytonomeryx scotti are the largest known homacodonts. 



P. progressus was originally described, as noted above, from the 

 Randlett or lower member of the Duchesne River formation, but 

 collections obtained by the U. S. National Museum from a quarry 

 in Myton pocket show that it is also represented in the Uinta C 

 fauna, and by considerably more material. 



MEASUREMENTS IN MILLIMETERS OF DENTITION IN SPECIMENS OF 



Pentacemylus progressus 



Length of upper cheek tooth series, C-M', inclu- 

 sive 45.6 



Length of upper cheek tooth series, anterior mar- 

 gin of alveolus for P* to posterior margin 

 of M* .... 40.70 



Upper premolar series, anterior margin of alveolus 

 for P^ to posterior margin of P* 



Upper molar series, M*-M^ inclusive 



C, anteroposterior diameter: transverse diameter. . 



P^ anteroposterior diameter : transverse diameter* 



P*, anteroposterior diameter : transverse diameter. 



23.0a .... 

 18.6 18.9 



4.0 : 2.5 



6.4:5.0 6.0:4.4 

 5.0:6.4 4-8: 5-7 



10 Also illustrated in Peterson, 1931b, fig. 9; and Scott, 1945, pi. i, figs. 5-6. 



