438 PATAGONIAN EXPEDITIONS: PAL/EONTOLOGY. 



No. 15,710. No. 9594. 



Length of molar series on alveolar border . . . . .010 



Width of base of incisor ........ .002 



Depth "< .003 



Posterior premolar, antero-posterior diameter .... .0012 



" " transverse "..... .001 



M T , antero-posterior diameter . . . . . . -00475 S 



" transverse " 0024 .0025 



My, antero-posterior " . . ' . . . . . . .003 .003 



" transverse " . . . . . . . . .002 .002 



MJ-, antero-posterior "........ .002 



" transverse " 0015 



Depth of mandible below first molar ...... .005 .005 



ABDERITINA1. 

 ABDERITES Ameghino. 



(Plate LXIV, Figs. 3, 30 ; Text Fig 9, a, b.) 



Abderites Amegh. ; Enum. Sist. Especies Mamif. F6s. Patagonia Austral, 

 p. 5, 1887. 



Diprotodont marsupials in which the first lower molar is developed as 

 a striated sectorial blade resembling superficially the sectorials of the Plagi- 

 aulacidse. 



Dentition (PI. LXIV, figs. 3, 3^ ; text figs. 9, a, b}. --The formula 

 for the lower dentition is ^-^> - v The median incisors are not preserved 

 in either specimen of Abderites crassignathus in the Princeton collection 

 (Nos. 15,079, 15,425), but, judging from the size of their alveoli, they were 

 large teeth. The posterior premolar is a very small, single-rooted, cylin- 

 drical tooth closely applied to the anterior root of MT. Between the 

 enlarged incisor and the posterior premolar are alveoli for four single- 

 rooted, vestigial teeth. The molar series is placed very obliquely to the 

 long axis of the jaw, more so than in any other member of the Caenoles- 

 tidae, the anterior blade of the first molar projecting beyond the plane of 

 the outer surface of the mandible. The molars are double-rooted and 

 decrease in size posteriorly, as in all the Caenolestidae. The first has been 

 converted into a very perfect sectorial by the complete obliteration of the 

 metaconid, the great elevation and lateral compression of the protoconid- 

 paraconid blade, and the development of parallel vertical ridges on both 

 the outer and inner faces of the crown. The ridges on opposite sides 



