22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



The combination of characters seen in the upper dentition is highly- 

 suggestive of Prota pirns and the possibility of an ancestral relation- 

 ship is not precluded by the characters of the lower dentition, as it so 



MEASUREMENTS IN MILLIMETERS OF DENTITIONS IN SPECIES OF 



Dilophodon 



D., cf. 



Icotanus 

 U.S.N.M. 



No. 

 21098 



Length of upper dentition, P'-M'' inclusive 39-0 ' 



Length of upper premolars, P^-P* inclusive, at alveoli 17.0* 



Length of upper molar series, M' (at alveolus) -M^ inclusive 23.2 



P', anteroposterior diameter : transverse diameter 5.7 : 8.2 



M^ anteroposterior diameter perpendicular to anterior margin 8.2" 



M^ transverse diameter across parastyle and protocone 9.8° 



M^ anteroposterior diameter perpendicular to anterior margin 8.7 



M*, transverse diameter across parastyle and protocone 10.4 



D. minus- D., cf. 



cuius Icotanus 



P.U. No. U.S.N.M. 



1 00 1 9 No. 



Type 20207 



Length of lower cheek tooth series, P2-M3 inclusive 46.6 41. i 



Length of lower premolar series, P2-P4 inclusive 18.7 16.2 



Length of lower molar series, M1-M3 inclusive 28.3 25.4 



P2, anteroposterior diameter : greatest transverse diam- 

 eter 5-6 : 3-4 4-8 : 3-5 



P3, anteroposterior diameter : greatest transverse diam- 

 eter 6.5 : 4-6 57 : 4-5 



Pi, anterioposterior diameter : greatest transverse diam- 

 eter 6.8 : 5.5 6.1 : 5-2 



Ml, anteroposterior diameter : transverse diameter of tri- 



gonid 8.1:5.2 7-2: 5-2 



Mt, anteroposterior diameter : transverse diameter of tri- 



gonid 94: 5-9 8.8:6.1 



M3, anteroposterior diameter : transverse diameter of tri- 



gonid 10.6:6.3 10.1:6.5 



* Estimated. 



clearly is in the isectolophid line. The phyletic position of Dilophodon 

 with respect to Protapirus had been suspected by Peterson (1919. p. 

 113) on the basis of the lower dentition, and the likelihood of such 

 a relationship seems greatly strengthened by information furnished 

 by the upper cheek teeth of Dilophodon, cf. Icotanus. 



