MAY, 1921. AMERICAN MARSUPIAL, CJENOLESTES OSGOOD. 119 



groove extending from the middle of its outer surf ace to the broad single 

 root. In the closely allied genus Orolestes recently described by Thomas, 

 the canine is said to be double-rooted, and shaped like a premolar. 

 Hence we have in the caenolestids now living a graded series showing a 

 change in the character of the canines from a two-rooted premolarif orm 

 condition to a single-rooted caniniform condition. That this is the 

 direction of the change and not vice versa seems fairly clear. In the first 

 place, the two-rooted condition is the more primitive. Moreover, 

 although it might be reacquired through "retrogression," as held by 

 Bensley, it is very unlikely that the reversal would be represented by 

 these particular stages. Thus, while the grooved root is the obvious 

 intermediate state in change from a double to a single root, it is scarcely 

 what would be expected if the direction of change were reversed. The 

 logical intermediate stage, if we were dealing with a retrogression from 

 a single to a double-rooted condition, would be a lengthening of the 

 crown with possible appearance of antero-posterior cusps before tend- 

 encies to division of the root began. Hence it seems that the change in 

 this case is toward rather than away from a long single-rooted piercing 

 type of canine. It is plainly a very recent specialization and is quite in 

 accord with the animal's present insectivorous and predaceous habits. 

 Of still further interest in this connection is the fact that this change 

 has progressed farthest in the northernmost form, that is, the one 

 presumably most distant from the parent stock. 



The lower canines have no intrinsically distinctive characters but 

 are quite like the incisors preceding them upon which they lie in semi- 

 prostrate position. 



UPPER PREMOLARS. 



The anterior premolar is small, greatly compressed laterally, and its 

 crown In side view is nearly triangular, its anterior outline being prac- 

 tically straight and its posterior slightly concave. In wholly unworn 

 condition, traces of a posterior cusp are evident but none of an anterior 

 one. It is situated about midway between the canine and the middle 

 premolar and is separated from each of these by a space equal to approxi- 

 mately one and one-half times its own length. 



The middle premolar is somewhat larger than the anterior, but is 

 similarly compressed. Its posterior outline is concave and sweeps back- 

 ward to a definite posterior cusp. It is separated from the posterior 

 premolar by a slight space about half as long as that between it and the 

 anterior premolar. 



The posterior premolar, which is probably a replacing tooth, is 

 separated by a slight space from the middle premolar. It is situated 



