874 THE WHITE RIVER FATHsTA. 



As compared with the brain of the rabbit {Lepus cuniculus) figured by 

 Leuret and Gratiolet/ that of the Palceolagus haydeni is distinguished by 

 the absolutely much smaller size of the hemispheres, and by the absolutely 

 larger olfactory lobes, the excess being in transverse dimensions and not in 

 the longitudinal. An important difference is also the absence of the 

 median posterior production of the hemispheres seen in the rabbit, the pro- 

 longation in the extinct species being lateral, and extending little behind the 

 Idbus hippocanqn. The indications of the convolutions of the superior sur- 

 face are similar in the two. 



As observed by Leidy, this genus presents the same number of teeth 



as in the existing rabbits, viz, I. - ; C.^; M. ^ ; and the difference con- 

 sists in the fact that the first molar possesses two columns, while in Lejms 

 there are three. Having collected a great number of remains of this genus, 

 I am able to show that it is only in the immature state of the first molar 

 that it exhibits a double column, and that in the fully adult animal it con- 

 sists of a single column with a groove on its external face. The dentition 

 undergoes other still more important changes with progressing age, so as 

 to present the appearance of difference of species at different periods. These 

 will be explained under the head of the P. haydeni, the most abundantly 

 repi-esented in the collections. It may be mentioned here that in neither P. 

 haydeni nor P. turgidus is there any evidence that more than two anterior 

 molars are preceded by deciduous teeth. The latter are present in many 

 specimens. 



Three species of this genus are known to have lived in Colorado during 

 the White River epoch of the Miocene. Bones of two of the species have 

 been found also in Dakota. The P. haydeni was probably the most abun- 

 dant mammal of the fauna of that period. 



Depth of ramus at penultimate molar, 9°"" ; length of molar series, 10°"" ; no third lobe 

 to molars P. haydeni. 



Depth of ramus at penultimate molars, 11"" ; length of tooth series, 14°"° ; a third 

 posterior lobe of the molars P. triplex. 



Depth of ramus at penultimate molar, 12-14"» ; length of tooth series, 13-16"™ ; no 

 third lobe P- turgidus. 



'Anatomie Compar^e du Syst^me Nervenx, PI. Ill, Figs. 1, 2. 



