384 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. [roLYL 



tinuous at both extremities, and extend with the external to the base of 

 the olfactory lobes. There is no definite indication of the Sylvian fis- 

 sure. The lohus Mpjjocampi i)ro trades laterally a little beyond the bor- 

 der of the external convolution. Its form is depressed. 



As compared with the brain of the rabbit {Lepus cunieuhis) figured 

 by Leuret and Gratiolet*,that of the Palaeolagus liaydeni is distinguished 

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

 solutely 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 iDroduction of the hemispheres seen in the rab- 

 bit, the prolongation in the extinct species being lateral, and extending 

 little behind the lohus Mppocamin. The indications of the convolutions 

 of the superior surface are similar in the two. 



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

 as in the existing rabbits, viz, I. f ; C. %; M. f ; and that the difference 

 consists in the fact that the first molar possesses two columns, while in 

 Lepus 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 consists 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 

 livydeni^ the most abundantly represented in the collections. It may be 

 mentioned here that in neither P. Jiaydeni 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. 



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

 the White Eiver epoch of the Miocene. Bones of two of the species 

 have been found also in Dakota. The P. liaydeni was probably the 

 most abundant mammal of the fauna of that period. 



Dei^tli of ramus at penultimate molar, 6"^™; length of luferior molar series, 10™"'; 



no third lobe to molars 1 P. agax^ttillus. 



Depth of ramus at penultimate molar, 9™™; length of molar series, 10™™; no third 



lobe to molars P. hai/deni. 



Depth of ramus at penultimate molar, 11mm . 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. 



LEPUS Linn. 



Dental formula : I. i ; C. § ; P-m. f ; M. f . First S'uperior molar sim- 

 ple ; first inferior molar with two external grooves ; last inferior molars 

 consisting of two cylinders. Postorbital processes present. 



I am acquainted with but one extinct species of this genus, and this 

 is from the Truckee or Middle Miocene period. It proves the ancient 



"Anatomie Compar^e du Systeme Nerveux, PL III, Figs. 1, 2. 



