339 



Measurements of seventeen skulls of URSUS AMERICANUS. 



The range of variation not dependent upon locality is more fully indi- 

 cated in the table of detailed measurements of these skulls given below, 

 but certain of the most prominent points of variation are not well shown 

 by any series of measurements. Especially is this the case in respect to 

 the amount of convexity different specimens present, in which individual 

 variation is strongly marked. One of the most prominent distinctions of U. 

 americanus as compared with U. arctos and its varieties is the great con- 

 vexity of the upper outline of the skull, both antero-posteriorly and trans- 

 versely. Another feature is the constriction of the facial portion, giving a 

 concave outline to the nasals when viewed in profile. But there are 

 exceptions, even to the first of these distinctions, one or two specimens 

 occurring (especially No. 2250 from New York) in which the flattening 

 of the frontal region is as marked as in average skulls of U. horribilis. 

 This flattening is also well marked in Nos. 1155 and 1156, from Louisiana. 

 The greatest convexity is reached in No. 3484, from Key Biscayne, Fla. ; 

 this and No. 2250 (New York) presenting the two extremes in respect to 

 convexity. No. 3' 94, from Georgia, has about the same degree of con- 

 vexity as the Florida specimen. No. 2250 is also remarkable for the 

 shortness of the facial portion of the skull, thereby imparting to it a 

 greater than the usual ratio of width to length. In this specimen 

 (mentioned by Professor Baird as remarkable for its ftidth*), the width 

 is 0.69 of the length. In another, from Louisiana (No. 1155), it falls as 

 low as 0.54 ! The average ratio of width to length is about 0.56 to 0.60. 



The teeth of U. americanus seem, in looking at them, to be relatively 

 much smaller than in V. arctos, but, upon careful measurement, the 

 difference is quite small, while they are of the same relative size as 

 those of U. horribilis. In U. americanus, the temporal ridges pass more 

 abruptly inward toward the medial line of the skull than in either U. 

 horribilis or 17. arctos. 



The most important distinction presented by U. americanus is the 

 form of the last upper molar. In U. americanus, the crown is widest at 

 the middle, narrowing both anteriorly and posteriorly, but most rapidly 

 posteriorly. The inner border is nearly straight ; the outer has a promi- 

 nent medial convexity, while in U. horribilis and U. arctos both outlines 

 are nearly straight and generally about equally convergent. In U. 

 americanus, the anterior third of the last molar is generally narrower 



'Main. N. Amer., p. 227. 



