1883. | The Extinct Rodentia of North America. 171 
mechanically perfect as in Lepus. The extremity of the fibula is 
coéssified, and the astragalar grooves are deep. This is a contri- 
bution to the evidence that the posterior feet of the Mammalia 
have advanced more rapidly in advancing evolution than the an- 
terior. As the posterior limbs furnish most of the energy, and 
therefore sustain most of the shocks in progression, there is 
doubtless a connection between the two facts, of cause and 
effect. 
A cast of the cranial chamber of a specimen of Paleolagus 
haydeni displays the superficial characters of the dram. As in 
the order generally, the hemispheres are small and are contracted 
anteriorly. The greater part of the cast of the cerebellum is lost, 
but enough remains to show that it was large. The olfactory lobes 
are large; they are not gradually contracted to the hemispheres, but 
expand abruptly in front of them, being separated by a constriction 
only. They are wider than long and than the exterior part of the 
hemispheres. Their cribriform surface is wide, and extends backwards 
on the outer sides. Traces of the three longitudinal convolutions 
can be observed on the hemispheres above the lobus hippocampi. 
The internal and median are continuous at both extremities, and 
extend with the external to the base of the olfactory lobes. There 
is no definite indication of the Sylvian fissure. The lobus hippo- 
campi protrudes laterally a little beyond the border of the exter- 
nal convolution. Its form is depressed. 
As compared with the brain of the rabbit (Lepus cuniculus) fig- 
ured by Leuret and Gratiolet, that of the Paleolagus haydent is 
distinguished by the absolutely much smaller size of the hemis- 
pheres, 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 prolongation 
in the extinct species being lateral, and extending little behind the 
lobus hippocampi. 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. ł; C. $; M: $; and that the 
difference consists in the fact that the first molar possesses two 
columns, while in Lepus there are three. Having collected a 
~*See On the effects of Impacts and Strains on the feet of Mammalia, AMER. 
PRALIST, 1881, p. 543. eae 
