MESODONTA. 235 



and stand on the external base of the crown. The third true molar does 

 not differ materially from the second. 



Though the species of this genus are not numerous, individuals of two 

 of them are exceedingly common in the Eocene beds of North America. 



The species of this genus known to me by their mandibles are five, and 

 these differ chiefly as follows: 



a. Posterior inner cusps of inferior molars, elevated. 



Length of true molars M. .0175 ; last molar elongate R. powellianus. 



Length of true molars M. .0165; last molar elongate R. lemoinianus. 



Length of true molars M. .0140; last molar longer than second H. paulus. 



Size as last; last molar shorter than second H. miticulus. 



Length of true molars M. .0115; last molar elongate . H. vicarius. 



aa. Heel of true molars i and ii, basin-shaped, without posterior inner cusp. 

 Length of inferior true molars M. .010; last molar as long as penultimate, .if. acolytus. 



Hygpsodus powellianus Cope. 



Plate XXIIId, figs. 3-4. 



The largest species of the genus is represented by more or less imper- 

 fect mandibular rami of eleven individuals, none of which unfortunately 

 support premolar teeth. The characters of the true molars are those of the 

 other species of the genus. 



The cusps alternate with each other, and are quite acute. The sections 

 of the inner cusps are nearly round, while those of the externals are cres- 

 centic. This is due to the fact that they send out descending ridges to the 

 inner side, one anteriorly, the other posteriorly. There is a small median 

 posterior tubercle, rather better marked than in Jff. paulus. A low ledge 

 connects the anterior cusps anteriorly, but there are no other cingula. The 

 last inferior molar is narrowed and produced posteriorly, and the edge of the 

 heel is elevated. Enamel entirely smooth. 



The anterior border of the masseteric fossa is distinct to about the 

 middle of the depth of the ramus, where it disappears. 



Measurements. 



M. 

 Length of premolars 0175 



Diameters of M.ii^^°*«'^''I'°^*«"''' 0055 



c transverse 0042 



Diameters of M-iii^^^t^^P"^**™' 0062 



I transverse 0045 



Depth of ramus at M. ii 0115- 



