1894.] 



NATURAL SCIENCES OP PHILADELPHIA. 



125 



their lack of value as specific characters may be readily estimated 

 from the following table, in which they are presented for each form, 

 in parallel descending series, showing at a glance the extent to which 

 each overlaps the others: — 



Nor is there more constancy in the details of tooth structure. On 

 the whole, arcto>< presents an upper hinder molar perhaps straighter 

 on the outer line and wider at its hinder extremity than is usual in 

 most of horribilis, but the, variations are great in both. Two adult 

 skulls of horrihilk, collected by me in Colorado, within fifty miles 

 of each other, exhibit almost the extreme of each form. 



The fourth upper premolar is apt to be longer in arctos and in 

 horribiUs there is more of a shelf- like projection of the cingulum at 

 its front inner corner, but hardly any two are alike and in each, ex- 



* Estimated ^ Young. 



