SINCLAIR: MARSUPIALIA OF THE SANTA CRUZ BEDS. 413 



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. 



Microbiotherimn 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, II- MA on alveolar border .021 



" anterior border of C-M . . . . . . . . .012 



" superior premolar series ........ -0035 



MI-MA 007 



" IT~ MI on alveolar border . . . . . . . . -0173 



C-M T .0135 



" inferior premolar series ........ .004 



" M T -M T on alveolar border ........ .0078 



Depth of mandible below anterior premolar ...... .004 



" " " M T 0045 



" " " M 5 0055 



Antero-posterior diameters of Mi, M^ and M* each . . . .002 



Transverse diameters of Mi and M3. each ..... .0022 



diameter " M-S. .002 



Antero-posterior diameter of MA . . . .0015 



" diameters of M T , M 7 and M 8 - each . .002 



Transverse diameters of M T , Mj and Mg- each . .0015 



Antero-posterior diameter of Mj . . .00175 



Transverse diameter of M ? ... . -OO i 2 



MlCROBIOTHERIUM GALLEGOSENSE Sp. nOV. 

 (Plate LXII, Figs. 3, 3*.) 



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 



