no. 2077. AN EXTINCT MARSUPIAL FROM FORT UNION— GIDLEY. 399 



seems rather to favor the view that the excessive number of molars 

 in Myrmecobius is due to "a simple reduplication of teeth from the 

 posterior portion of the dental lamina/' the minute size of the 

 molars and the great lengthening of the jaws offering just the condi- 

 tions favorable for an intercalation of new teeth. In defense of 

 this he observes that "even assuming a retention of the deciduous 

 teeth, we would still have to account for the occasional presence of 

 an additional lower molar." 1 The application of this observation, 

 however, is not clear, since, beginning with the normal marsupial 

 dental series, viz., p-^, dpj, ra«, it requires but the addition of two 



more permanently retained deciduous molars to equal the greatest 

 number of post-canine teeth found in this species, namely, 9, making 



the dental formula for the lower j aw as follows : 1 ^ C J> P~£> ^Pg* m 2 or 3. 



This would still leave two teeth less than the normal combined num- 

 ber of the milk and permanent series of post-canine teeth found in 

 both the marsupials and placentals, the missing ones being dp t of 

 the first series, probably very early shed or never replaced, and p 4 

 of the permanent premolar series, either early absorbed or never 

 developed. The variability in the molar series in Myrmecobius seems 

 due to the presence or absence of the last molar, probably a disap- 

 pearing tooth. In the upper jaw the last molar seems to be nor- 

 mally wanting, while the second is apparently in the process of dis- 

 appearing, being sometimes present and sometimes wanting. There 

 is also an occasional variation in the number of milk molars retained, 

 the upper jaw of a specimen in the United States National Museum 

 collection having two such teeth on one side and only one on the 

 other. 



However regarded, the teeth of Myrmecobius, as pointed out by 

 Bensley, show every indication that the genus was derived from a 

 primitive form with normal tritubercular teeth of the general insectiv- 

 orous type. The present specialization is toward a pseudotricono- 

 dont type, evidently acquried through the peculiar development of 

 the inner and the atrophy of the outer cusps of the lower molars, with 

 a similar but reverse modification of the upper molars, with the addi- 

 tion to the series of supernumerary teeth accomplished through the 

 retention of milk molars. Conceding this to be the true history of 

 the development of Myrmecobius, the little lower jaw from the Fort 

 Union formation, whether considered ancestrally related or not, is 

 morphologically intermediate in nearly every particular between such 

 a jaw as that of Myrmecobius and those of the generalized primitive 

 types of trituberculate mammals. It stands nearer to the tritubercu- 

 lar form, it is true, but is nevertheless intermediate in development, 



1 Trans. Linn. Soc. London, ser. 2, Zool., vol. 9, pt. 3, 1903, p. 100. 



