41 4 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 
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 My on alveolar border 
“ of premolar series on alveolar border . 
“ “ molar “ “ “ “ 
Antero-posterior diameters of My and My each 
Transverse “ “ “ “ “ “ 
Depth of mandible below median premolar 
Microbiotherium tehuelchum Ameghino. 
(Plate LXII, Figs. 4, 4 a, 6-12.) 
Microbiotherium tehuelchum Amegh. ; Enum. Sist. Especies Mamif. Fos. 
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. gallego sense, 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 
.016 
.006 
.010 
.0025 
.002 
.005 
.006 
.007 
