12 PLEISTOCENE MAMMALIA. 



(6) The shape of the forehead; and 



(7) The form and size of the mastoid process of the periotic. 



Although the conclusion arrived at is, that it is impossible to separate II. 

 horribilis (ferox) from U. arctos by the study of the skeleton, it has been thought 

 advisable in the tables of comparative measurements to quote without comment the 

 name previously assigned to any specimen. 



b. Dentition (Plate VI). 



(1) Distinctive Features of the Teeth in the Genus Ursus. — The dental formula is 

 i. f, c. x, pin. \, m. f, as in the dog. In Hyaena it is i. §, c. T , pm. |, m. T , and in 

 Felis i. f, c. \, pm. f, m. T . Although the upper incisors increase somewhat 

 in size from the first to the third, Ursus agrees with Felis and differs from Hyaena 

 in presenting a marked contrast in size between c. and i 3. The canine is dis- 

 tinguished from that of the lion by the more massive character of the root. The 

 three anterior premolars above and below are very small, one-rooted, and often 

 early deciduous, especially the second, which is rarely present in the adult animal. 

 Pm. 1 is situated close to the canine, pm. 3 close to pm. 4, which is the upper 

 carnassial. This tooth lacks the antero-internally placed inner tubercle supported 

 by a distinct root, which is so characteristic of Felis, Hyaena, and Canis. Pm. 4 

 possesses, however, a postero-internally placed inner cusp which, as in other Ursidas, 

 is not supported by a distinct root. The sectorial characters of pm. 4 are very 

 little marked, and it is much smaller than m. 1. The crowns of both the upper 

 true molars are longer than broad, and have flattened tuberculated grinding 

 surfaces; the second has a large backward prolongation or heel. The lower 

 carnassial (m. 1) has a small and indistinct blade and a greatly developed 

 tubercular heel. The second molar is of about the same length as the carnassial, 

 but with a broader and more flattened tubercular crown. The third is smaller. 

 The milk-teeth are comparatively simple and shed at an early age. 



The following descriptions are of teeth of Ursus spelaeus, but in each case the 

 differences presented by the teeth of bears of the arctos type are noted. 



It has been thought best when describing the teeth not to use terms involving 

 assumptions of homology and requiring long explanatory prefixes. The terms 

 cusp and tubercle are regarded as synonyms for small elevations on the surface of 

 a tooth. The terms cone or lobe are used as denoting a rather larger elevation, and 

 the terms talon or heel for posteriorly placed segments of a tooth. 



(2) Permanent Dentition of the Upper Jaw (see PI. VI). — I. 1 and 2 are very 

 similar teeth showing a prominent anterior, pointed, and somewhat backwardly 

 directed cone or cusp, and a depressed triangular posterior area not bearing any 



