414 PATAGONIAN EXPEDITIONS: PAL/EONTOLOGY. 



are double rooted. The median and posterior premolars are slightly 

 spaced. No trace of an external cingulum can be detected on the molars. 

 Like M. tortor, the lower margin of the masseteric fossa is separated from 

 the inferior mandibular border by a broad convex surface. The mandible 

 is moderately convex below and increases regularly in depth posteriorly. 

 A minute foramen, communicating presumably with the inferior dental 

 canal, pierces the masseteric fossa anteriorly. It is not present in M. 



tortor and M. tehuelchum. 



MEASUREMENTS. 

 Length, anterior premolar to M T on alveolar border . . . . . .016 



" of premolar series on alveolar border ...... .006 



" " molar " " " " oio 



Antero-posterior diameters of My and Mj- each ..... .0025 



Transverse " << OO2 



Depth of mandible below median premolar ...... .005 



" " " " M T 006 



" " " " My . . . .007 



MlCROBIOTHERIUM TEHUELCHUM AmeghinO. 



(Plate LXII, Figs. 4, 401, 6-12.) 



Microbiotherium tehuelchum Amegh. ; Enum. Sist. Especies Mamif. F6s. 

 Patagonia Austral, p. 7, 1887. 



The left half of a lower jaw associated with two upper molars and parts 

 of the skull and skeleton (No. 15,038) have been identified with this spe- 

 cies. The principal features of the skull and skeleton have already been 

 noticed in the generic diagnosis and do not call for further comment. 



The species may be recognized by its size, which is smaller than in M. 

 tortor, by the less prominent chin, the long vertically-directed lower 

 canine, the well-spaced premolars, the strongly inflected angle and the 

 close approximation of the lower margin of the masseteric fossa to the 

 inferior border of the jaw. The mandible differs considerably in shape 

 from that of M. tortor and M. gallegosense, having the angle inclined 

 obliquely inward and upward. An antero-external cingulum is devel- 

 oped on the lower molars. The upper teeth have the external styles 

 rather more prominent than in M. tortor. 



MEASUREMENTS. 



Length, anterior border of canine to My inclusive .... .014 



inferior premolar series ........ .005 



