AN EXTINCT MARSUPIAL FROM THE FORT UNION WITH 

 NOTES ON THE MYRMECOBIDAE AND OTHER FAMILIES 

 OF THIS GROUP. 



By James Williams Gidley, 



Assistant Curator of Fossil Mammals, United States National Museum. 



In a small lot of Fort Union mammals recently collected by Mr. 

 A. C. SilberKng in Sweet Grass County, Montana, there is a lower jaw 

 which, though small in size, is of great importance, since it apparently 

 represents an unexpectedly early appearance of a possible relative 

 of the Myrmecobidae a family of marsupials hitherto known only from 

 a single living genus, Myrmecobius. The description given below is 

 followed by a brief discussion of the peculiar features of the species, 

 its possible affinities, and some short notes on the probable deriva- 

 tion of the marsupials in general. 



MYRMECOBOIDES, new genus. 1 



This genus, represented by a single species of small size, may be 

 distinguished as follows: Canine semipremolariform, being irregu- 

 larly triangular in cross section and but slightly curved; canine and 

 the three simple premolars evenly spaced with short intervening 

 diastemae. There is also a short diastema between the canine and i 3 

 (the position of the other incisors is not known). Fourth tooth 

 behind the canine (probably dp 4 retained) completely molariform; 

 true molars tri tubercular, with well-developed basin heel, but with 

 inner cusps of trigonid (paraconid and metaconid) as high or higher 

 than main outer cusp (protoconid). 



Ordinal affinities of this genus marsupialian, and it is probably 

 related to the Myrmecobidae. 



MYRMECOBOIDES MONTANENSIS, new species. 



Plate 23. 



Type-specimen— (Cut. No. 80,37, U.S.N.M.). A left lower jaw 

 carrying a series of 8 teeth, c to m 3 . Collected by A. C. Silberling. 



Type-locality and horizon. — Sweet Grass County, Montana, "Gidley 

 Quarry," about the middle of the Fort Union deposits of that 

 locality. 



i This name is given to the ancient form on account of its likeness to Myrmecohius rather Hum as a 

 positive assumption of real relationship. 



Proceedings U. S. National Museum, Vol. 48-No. 2077. 



395 



