60 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 28 



of Kennedy's Hole and Devil's Playground. Kennedy's Hole type 

 specimen of P. minor would appear to be from a distinctly earlier 

 horizon than the Myton pocket material. I have no doubt but that 

 P. petersoni was derived from P. minor and that when more material 

 of these small protoreodonts is found, presumably at intervening hori- 

 zons, no clear-cut separation will be feasible and that, as in the 

 sequence of large protoreodonts (P. pumilus and P. pum,ilus annec- 

 tens), P. petersoni may come to be regarded as an advanced variant 

 of P. minor. At present, however, it would appear that P. petersoni 

 has actually progressed somewhat farther from its antecedent, P. 

 minor, than its contemporary P. pumilus annectens has from P. 

 pumilus (sensu stricto). 



In general, the skull of Protoreodon petersoni is like that of P. 

 pumilus. Other than size, there is little in their form to distinguish 

 them. It is noted, however, that the postorbital processes are better 

 developed in P. ^petersoni, although they do not close behind the orbit 



MEASUREMENTS IN MILLIMETERS OF DENTITION IN TYPE SPECIMEN OF 



Protoreodon petersoni, p.u. no. 14404 



Length of upper cheek tooth series, C (at alveolus )-M^ inclusive 50.5 



Length of upper cheek tooth series, P*-M', inclusive 42.0 



Upper premolar series, P'-P^ inclusive 21.8 



Upper molar series, M^-M*, inclusive 21.5 



C, anteroposterior diameter (at alveolus) : greatest transverse diame- 

 ter 4-9 : 4-7 



P\ anteroposterior diameter : greatest transverse diameter 5-5 : 2.8 



P^ anteroposterior diameter : transverse diameter* 5-5 : 3-9 



P*, anteroposterior diameter : transverse diameter 5.6 : 5.0 



P*, anteroposterior diameter : transverse diameter 5-3 : 6.7 



M\ anteroposterior diameter : transverse diameter* 6.5 : 7.9 



M^ anteroposterior diameter : transverse diameter 7.9 : 9.8 



M^ anteroposterior diameter : transverse diameter 8.3 : 10.4 



Length of lower cheek tooth series, Pi (at alveolus) -M3, inclusive 46.4 



Lower premolar series, Pi, (at alveolus) -P4, inclusive 22.4 



Lower molar series, M1-M3, inclusive 24.3 



Pi, anteroposterior diameter (at alveolus) : greatest transverse diame- 

 ter 4.8 : 3.8 



Pa, anteroposterior diameter : greatest transverse diameter 5-3 : 2.3 



P3, anteroposterior diameter : greatest transverse diameter 6.2 : 3.0 



P4, anteroposterior diameter : greatest transverse diameter 6.5 : 4.6 



Ml, anteroposterior diameter : transverse diameter of talonid 6.2 : 5.0 



Ma, anteroposterior diameter : transverse diameter of talonid 7-3 • 5-8 



Mj, anteroposterior diameter : transverse diameter of trigonid 1 1.4: 5.9 



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

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

 anteroposteriorly perpendicular to anterior margin and transversely across anterior portion 

 of tooth. 



