no. 2077. AN EXTINCT MARSUPIAL FROM FORT UNION— GIDLEY. 397 



main outer cusps (protoconid and hypoconid, respectively) in height. 

 The relatively small size of the latter, and especially the protoconid, 

 which is apparently diminishing, is a modification toward the. con- 

 dition reached in Myrmecobius, and away from the usual development 

 of primitive tri-tuberculate teeth. In fact, all these characters denote 

 progressive development away from the type characteristic of the 

 primitive insectivores, carnivores, and creodonts, and toward 

 Myrmecobius. 



A point of difference is the very close approximation of the para- 

 conid to the metaconid. Although quite distinct at the summit, 

 the paraconid forms a part of the elevated metaconid column. 

 In Myrmecobius this cusp is usually distinct and directed forward 

 but as pointed out by Dr. B. Arthur Bensley 1 in his valuable contri- 

 bution on The Evolution of the Australian Marsupialia, the dentition 

 in the living genus is exceedingly variable in detail. One of the 

 two specimens in the U. S. National Museum collection shows a much 

 closer approximation to Myrmecoboides in this respect than does the 

 other ; hence it is possible that the more anterior position of the para- 

 conid in Myrmecobius may have been secondarily acquired as a result 

 of the extreme lengthening of the jaw. Another point of difference 

 between the living and the extinct forms is the relatively greater ele- 

 vation and better definition of the trigonid in the latter. This, 

 however, is doubtless due to* its more primitive condition. 



The first molariform tooth in Myrmecoboides montanensis, which 

 stands in the position of p±, is worthy of special notice. It differs 

 in some important features from those behind it, but is so typically 

 and completely molariform as to suggest a true molar or more proba- 

 bly a tooth of the deciduous series. However, it is not possible to 

 say in the absence of proof whether it may not be after all a highly 

 specialized molariform premolar. Its very complex structure in 

 contrast with p 3 , which is a simple, unspecialized tooth, is against 

 this supposition and strengthens the view that it is more probably 

 a retained milk molar. Special features of the tooth itself favor this 

 conclusion and seem to preclude the other altenative of considering it 

 also a true molar. It has the same number of cusps as the molars, 

 and these, with the exception of the paraconid, have the same gen- 

 eral form, proportions, and arrangement. The crown is propor- 

 tionally narrower, the talonid is relatively smaller, and the large 

 paraconid is directed well forward, making up the whole anterior 

 portion of the trigonid, and is quite distinct. In this respect this 

 tooth differs markedly from the true molars. 



It thus appears that in this specimen we have real evidence con- 

 firming the view held by Winge and supported by Lydekker, Bensley, 

 and others that the first molariform tooth in all the marsupials is a 



i Trans. Linn. Soc. London, ser. 2, Zool., vol. 9, pt. 3, 1903, p. 102. 



