1910.] Microbiotherium, Peratherium. 205 



The question as to the phijIogejieUc validity of this series is raised by the 

 scarcity of pala^ontological evidence and by the general fact that supposedly 

 evolutionary series drawn from existing faunas have often proved erroneous. 

 The following brief review of the more important fossil Polyprotodonts 

 seems to lend further support to Dr. Bensley's conclusions. 



Fossil Poli/protodontia. 



In the Santa Cruz (Miocene) deposits of Patagonia are found certain 

 minute Polyprotodonts {Microbiotheriimi Ameghino) which are comparable 

 in size to some of the smaller South American opossums. In Sinclair's 

 description (1901, pp. 408-415) of Microbiotherium he shows that although 

 "placed by Ameghino in a separate family, the Microbiotheridje, this genus 

 possesses so many important characters in common with the Didelphidse 

 that the propriety of its reference to the latter family seems beyond question." 

 The chief peculiarity of Microbiotherium is the elongation of the premaxil- 

 laries and the extreme posterior position of the canine. These features 

 may indicate partly insectivorous habits. The dental formula is the same 

 as in Didelphis. The upper molars are tritubercular, with rounded para- 

 cone and metacone and Sinclair observers that they resemble closely the 

 molars of some of the subspecies of Caluromijs (the "Philander" opossimi). 

 The external cingulum and metastyle spur is more reduced than in the 

 living opossums and the molars may thus represent an early stage in the 

 development of the bunodont omnivorous molar. The lower molars of one 

 species retain the antero -external cingulum seen in Didelphis. The palatal 

 vacuities are very large. The skeleton is of Didelphid type with certain 

 Dasyurine pecuUarities (Sinclair). 



In the Oligocene of North America and Euro})e occurs another minute 

 Polyprotodont genus Peratherium Aymard described by Cope (1884, p. 789). 

 This also has the dental formula of Didelphis (at least in D.fugax, fide Cope). 

 The upper molars are tritubercular with V-shaped para- and metacones, the 

 metacone slightly enlarged, and there is a pronounced external cingulum. 

 The protocone is well developed and pointed. The nasals spread widely 

 posteriorly. Peratherium is also reported from the Upper Eocene of Eng- 

 land and France. In Peratherium {" Didelphys") aymardi Filhol,^ a small 

 species in which the lower jaw is less than half as large as in D. virginiana, 

 the lower premolars are less elongate and the space between the canine and 

 mj is shorter than in Didelphis {i. e., primitive characters). The coronoid 

 is broadly triangular, the broad angle is very prominent in external view. 

 The mental foramina are below p^ and m^, about as in Didelphis. 



1 Ann. Sc. G6ol., tome 8, 1877, p. 251, fig. 387. 



