36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 128 



Specific characters. — Comparable in size with contemporary Penta- 

 cemylus progressus, but with cheek teeth sHghtly smaller. Other 

 characters not distinguished from those cited for the genus. 



Discussion. — In addition to the particular characters of the upper 

 molars distinguishing Mytonomeryx, it was noticed that the type of 

 M. scotti exhibits a strikingly elongate snout with marked spacing 

 of the premolars between the canine and P3 above and below. From 

 the molar region forward the elongation is somewhat greater than in 

 the larger-toothed Pentacemylus progressus, and relatively greater 

 than in the smaller Pentacemylus leotensis. Some difficulty has been 

 experienced in distinguishing between lower jaws of Mytonomeryx 

 scotti and Pentacemylus leotensis, as the latter likewise shows a spac- 

 ing of the premolars. Differences were noted between the two, such 

 as broader molars, somewhat less reduced size of premolars, possibly 

 better-developed parastylid of lower premolars, somewhat greater 

 length of premolar series, and the absence of a diastema between P3 



MEASUREMENTS IN MIIXIMETERS OF DENTITION IN TYPE SPECIMEN OF 



Mytonomeryx scotti, u.s.n.m. no. 20401 



Length of upper cheek tooth series, C-M^, inckisive 49-70 



Length of upper cheek tooth series, P'-M^, inclusive 43-ifl 



Upper premolar series, P^-P^ inclusive 25.8a 



Upper molar series, M^-M^, inclusive 17.9 



C, anteroposterior diameter (at alveolus) : greatest transverse diameter. 3.8 : 2.8 



PS anteroposterior diameter : greatest transverse diameter 4.3 : 1.8 



P', anteroposterior diameter : transverse diameter* 6.0 : 4.6 



P*, anteroposterior diameter : transverse diameter 4.7 : 5.8 



M*, anteroposterior diameter : transverse diameter* 5-9 : 7-3 



M', anteroposterior diameter : transverse diameter 6.3 : 7.8 



M', anteroposterior diameter : transverse diameter 6.2 : 7.5 



Length of lower cheek tooth series, C (at alveolus )-M3, inclusive 50.1 



Length of lower cheek tooth series, P1-M3, inclusive 46.0 



Lower premolars, P1-P4, inclusive 26.4 



Lower molars, M1-M3, inclusive 19.7 



C, anteroposterior diameter (at alveolus) : greatest transverse diameter. 2.7: 2.1 



Pi, anteroposterior diameter : greatest transverse diameter 3.6 : 2.2 



P2, anteroposterior diameter : greatest transverse diameter 5.0: 2.1 



Pa, anteroposterior diameter : greatest transverse diameter 5.7 : 2.6 



P4, anteroposterior diameter : greatest transverse diameter 5-9: 3-6 



Ml, anteroposterior diameter : transverse diameter of talonid 5-7 : 4-3 



M2, anteroposterior diameter : transverse diameter of talonid 6.0 : 4.6 



Ms, anteroposterior diameter : transverse diameter of trigonid 8.5 : 4.6 



a, Approximate. 

 _ • Measurements of posterior upper premolars are taken anteroposteriorly across outer por- 

 tion and transversely perpendicular to outer margin. Those of upper molars are taken 

 anteroposteriorly perpendicular to anterior margin and transversely across anterior portion 

 of tooth. 



