r^' 



374 



MONOGllAPnS OF NOUTQ AMUKICAN RODENTIA. 



deposits of Dakota, and by Professor Ct)pc from those of Colorado. " Tlio 

 fossils indicate," says Dr. Leidy, "a species rather less in size than the Gray 

 Rabbit {Lvpus sijlvatlcus). Tiiey vary among tliemsclves in size and robust- 

 ness ; but, independently of what may be considered as individual peculiarities, 

 tiie dilfercnces 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 api)ear proportionately more robust. In a corresponding posi- 

 tion of the jaw there does not exist in the fossils tiie reticular foramina 

 oi)servcd in the vicinity of the mental foramen in the Rabbit." AVliile 

 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 tiiis genus, lie 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 tlie fully adult animal it consists of a single column with n 

 groove on its external face. The dentition undergoes other still more im- 

 ])ortant 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 tiiose containing a part 

 of the temporary molars to old specimens in which the teeth arc very much 

 worn. Professor Cope states that he has examined several hundred specimens 

 of tills species collected in Colorado, and gives a detailed account of the 

 changes in dental characters resulting from age. The earliest dentition known 

 to liim, he says, is the presence of two deciduous molars, the first and second in 

 position, wiiicii 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 ciisps 

 ,ich ; the second is similar, excepting that the simple cusp is at the posterior 

 :;nd of the tooth. Tiie grooves separating the lobes descend into the alveolus 



on the outer side, but stop above it on the inner In the next stage, 



the third permanent molar is projected, and has, like the second deciduous, n 



• Auu. Rvp. IJ. 8. Geol. Surv. ToiT. for 1873 0^4), pp. 477, 478. 



