ot4 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
deposits of Dakota, and by Professor Cope from those of Colorado. “The 
fossils indicate,” says Dr. Leidy, ‘‘a species rather less in size than the Gray 
Rabbit (Lepus sylvaticus). They vary among themselves in size and robust- 
ness; but, independently of what may be considered as individual peculiarities, 
the differences appear to depend mainly on difference of age. In regard to 
the form of the jaw-fragments, consisting merely of the portions containing 
molar teeth, they agree nearly with the corresponding portions in the Rabbit, 
except that they appear proportionately more robust. Ina corresponding posi- 
tion of the jaw there does not exist in the fossils the reticular foramina 
observed in the vicinity of the mental foramen in the Rabbit.” While 
having the same number of teeth as the existing Hares, they were at first 
supposed to differ somewhat in the constitution of the first lower molar, 
which Dr. Leidy described as composed of merely a double column instead 
of a triple column. Professor Cope,* however, states that, having collected 
a great number of the remains of this genus, he is “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. Tie dentition undergoes other still more im- 
portant changes with progressing age, so as to present the appearance of 
difference of species at different periods.” The roots of the lower incisors also 
penetrate considerably more posteriorly than in Lepus. Dr. Leidy describes 
in detail some ten or more fragments, representing both upper and lower 
jaws, and figures six specimens of varying ages, from those containing a part 
of the temporary molars to old specimens in which the teeth are very much 
worn. Professor Cope states that he has examined several hundred specimens 
of this species collected in Colorado, and gives a detailed account of the 
changes in dental characters resulting from age. The earliest dentition known 
to him, he says, is the presence of two deciduous molars, the first and second in 
position, which appear before any of the permanent series. “ Each of these 
has two roots, and the crown is composed of three lobes. In the first, the 
first lobe is a simple cusp; the two following are divided into two cusps 
each ; the second is similar, excepting that the simple cusp is at the posterior 
end of the tooth. The grooves separating the lobes descend into the alveolus 
on the outer side, but stop above it on theinner. .... In the next stage, 
the third permanent molar is projected, and has, like the second deciduous, a 
* Ann. Rep. U.S. Geol. Sury. Terr. for 1873 (1874), pp. 477, 478. 
