Sinclair: marsupialia of the santa cruz beds. 
4i3 
15,698) from the Lower Santa Cruz beds at Killik Aike. The principal 
cranial and dental characters have already been noted in the discussion of 
the genus and need not be repeated. 
Microbiotherium tortor is somewhat larger than M. tehuelchum and 
may be readily distinguished by the strong mandibular symphysis, which 
terminates inferiorly in a prominent tubercle, by the blunt anteriorly di- 
rected lower canine, the absence of spacing between the lower premolars, 
the less strongly inflected angle, and the wide separation of the lower 
borders of the masseteric fossa and angle. 
Measurements. 
Length, I-T- on alveolar border ....... 
“ anterior border of C-M-i ....... 
“ superior premolar series ....... 
“ Mi-Mi- 
“ Iy-My on alveolar border ....... 
“ C-My 
“ inferior premolar series ....... 
“ My-My on alveolar border ....... 
Depth of mandible below anterior premolar ..... 
****** ** M t 
**.**** '‘My 
Antero-posterior diameters of Mi, Mi and Mi each .... 
Transverse diameters of Mi and Mi each ...... 
“ diameter “ Mi ........ 
Antero-posterior diameter of Mi ....... 
“ diameters of My, My and My each .... 
Transverse diameters of My, My and My each ..... 
Antero-posterior diameter of My ....... 
Transverse diameter of My . . . . . . 
Microbiotherium gallegosense sp. nov. 
(Plate LXII, Figs. 3, 3 a.) 
This is the largest known species of Microbiotherium. The type speci- 
men (American Museum No. 9591), collected by Mr. Barnum Brown, on 
the Rio Gallegos, is the right ramus of a mandible, broken at both ends. 
The posterior premolar and the first two molars are preserved. The 
remaining premolars and molars are missing. 
The alveoli of the anterior and median premolars are closely crowded 
and the former tooth was directed obliquely to the axis of the jaw. Both 
.012 
.0035 
.007 
.0173 
•0135 
.004 
.0078 
.004 
.0045 
.0055 
.002 
.0022 
.002 
.0015 
.002 
.0015 
.00175 
.0012 
