398 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 48. 



fourth milk molar which is never replaced by a premolar as in the 

 placentals. The development of a fourth molariform tooth and the 

 loss of a premolar, however accomplished, must be considered one 

 of the earliest specializations of the marsupials since this reverse 

 tooth formula is characteristic of the entire group. 



An apparent objection to considering the Fort Union species as in 

 any way related to the ancestral line which gave rise to Myrmecobius is 

 the fact that it possesses only the normal number of post-canine 

 teeth, namely, seven, while Myrmecobius has eight or frequently nine 

 in the lower jaw. The importance of this difference however, de- 

 pends entirely on the source of the supernumerary teeth in Myrme- 

 cobius. If this characteristic is an ancient survival resulting from the 

 derivation of Myrmecobius from some one of the Jurassic mammal-like 

 forms having more than the normal number of post-canine teeth, as 

 suggested by Owen, Thomas, Leche, and others, then such a form as 

 Myrmecoboides could not be placed in the line of descent. But if, on 

 the other hand, as seems more probable, the increase in number of 

 teeth in Myrmecobius came about secondarily through the permanent 

 retention of deciduous premolars, as suggested by Winge, there is 

 nothing inconsistent in considering the Montana species an ancestral 

 relative of the later genus. A significant point in this connection is 

 the fact that in the M. montanensis jaw the longest diastema is be- 

 tween p 3 and dp A . 



ADDITIONAL NOTES ON THE ORIGIN OF MYRMECOBIUS. 



There is in our present knowledge nothing to support Owen's 

 hypothesis regarding the derivation of Myrmecobius. On the other 

 hand, as has been pointed out especially by Bensley, 1 there is consid- 

 erable evidence for, and a reasonableness in assuming that this genus 

 has, like the other marsupials, descended from an ancestral form with 

 a normal primitive marsupial dentition. In consequence of its ant- 

 eating habits, resulting in the modification of the entire series as 

 well as the great lengthening of the jaws anteriorly, the condition 

 was acquired whereby a second and finally a third additional tooth 

 of the remaining milk series were retained permanently, in the 

 manner suggested by Winge. 



The dental characters of the genus Myrmecobius have been 

 very fully described and discussed and the literature on the 

 subject reviewed by Bensley. Hence it is not necessary to enter 

 into much detail here. A few additional points suggest themselves, 

 however, and some of those advanced by Bensley and others are 

 seemingly somewhat altered by a restudy of the lower jaw of 

 Myrmecobius, when comparing it with the Montana specimen. While 

 not entirely disregarding the theory advanced by Winge, Bensley 



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



