FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 117 



The fact that Myrmecohoides is not a marsupial obviously deprives 

 it of any bearing on whether the fourth cheek tooth of marsupials is 

 dm4 or Ml, a question that Gidley properly discussed when under a 

 misapprehension as to the affinities of the genus. It also removes this 

 much support from Gidley's thesis, which was, however, also sup- 

 ported by other evidence, that the Australian families were differen- 

 tiated outside Australia and at a very early date.^* 



Among the Leptictidae, Myrmecohoides is aberrant and does not 

 belong with such typical genera as Diacodon or Leptacodon, although 

 its less direct relationship with them is highly probable. 



MYRMECOBOIDES MONTANENSIS Gidley 



Figures 17, 18 

 Myrmecoboides montanensis Gidley, 1915, p. 395. 



Type. — U.S.N. M. no. 8037, left lower jaw with canine and P1-M3. 

 Collected by A. C. Silberling. 



Horizon and locality. — Gidley Quarry, Fort Union, Middle Paleo- 

 cene horizon, Crazy Mountain Field, Mont. 



Diagnosis. — Sole known species of genus. Dimensions below. 



Remarks. — Gidley (1915) has accurately described the lower denti- 

 tion, and the slightl}^ different emphasis demanded by transfer to this 

 family is supplied by the revised generic diagnosis. The second spec- 

 imen mentioned by Gidley is U.S.N.M. no. 9418, which has only Mi.2 

 and these so worn that the cusp structure cannot be made out. 



U.S.N.M. no. 9552 is a right upper jaw with P'^-M^ that is so well 

 suited to be the upper dentition of Myrmecoboides montanensis, and 

 not of any other knowTi species in the fauna, that it may be referred 

 here. Dr. Gidley has noted that this is probably the upper dentition 

 of Myrmecoboides — further evidence of his later recognition of the 

 affinities of the genus, for this upper jaw is entirely leptictid in char- 

 acter and does not at all resemble Myrmecobius. These upper teeth 

 are very close to those of Prodiacodon throughout. P* seems to have 

 had the paracone and metacone better separated in Mrjrmecohoides, 

 but is imperfect in this region. M^ has the external shelf more pro- 

 nounced, the parastylar and metastylar lobes more projecting, a deep 



" When Gidley wrote, it was a reasonable a priori assumption that diverse marsupials would be the micro- 

 faunal elements most likely to appear in the Paleocene, although subsequent discovery has shown this not 

 to be the case. He was inevitably predisposed toward this view (as was also Matthew and as were other con- 

 temporaneous students), and his preliminary note on Myrmecoboides suffered from this preconception. It 

 is further exemplified by his manuscript notes on severa 1 other placental genera labeled as "Marsupial No. 1" 

 and so on; it has not seemed necessary to cite this first impression in each case, as Dr. Gidley would certainly 

 have abandoned it before completing his studies. Dr. Gidley also had a strong and more personal predis- 

 position, strikingly exemplified in the Myrmecoboides paper, to believe that the modern mammalian famDies 

 were of extremely remote origin. This is a legitimate thesis, and the erroneous nature of part of the supposed 

 evidence does not remove the possibility or vitiate Gidley's whole argument. His conclusion unquestion- 

 ably contains an important truth, but I believe, apparently with the consensus of recent students, that he 

 overemphasized its importance and extent. 



