130 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



Besides the didelphids and dasyurids there are at least two other 

 groups which may have retained some of the characters of the generalized 

 marsupial and which may throw some light on the origin of diprotodonty . 

 These are the Myrmecobiidae and the Peramelidae, the former group 

 with only one living species and the latter with a considerable number. 

 Myrmecobius, as is well known, has a very remarkable dentition com- 

 bining an excessive number of molars and a relative degeneration of all 

 its teeth. This apparently incompatible condition has been explained in 

 at least three ways. Owen, Thomas, Leche and others believed it to be a 

 primitive survival from ancestral forms having numerous post-canine 

 teeth such as Dryolestes of the North American upper Jurassic. Winge 

 has advanced the hypothesis that it is due to the retention of teeth 

 originally belonging to the deciduous series. In this he is followed by 

 Gidley (1915), who points out that "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, nine." A 

 third explanation is offered by Bensley who believes that the extra 

 molars of Myrmecobius are due to a "simple reduplication of teeth from 

 the posterior position of the dental lamina" induced by "the favorable 

 conditions of increased space in the molar region." This conclusion is 

 obviously influenced by the reduced incisor formula of Myrmecobius 

 which he believes is derived through the dasyurids from the didelphids 

 which have a more primitive number of incisors. But he fails to take into 

 account the fact that similar reductions of incisors have taken place in 

 the early history of marsupials in Tertiary or even Mesozoic times. 



To the writer, these different explanations seem equally theoretical 

 and the original one of Owen, if stated in general terms, is not less probable 

 than the others; that is, the resemblances between Myrmecobius and 

 Mesozoic forms seem quite as great as those between it and the modern 

 forms and, since other considerations point to great antiquity for several 

 different types of marsupials, there would seem to be no great improb- 

 ability in the belief that the extra molars are a survival rather than a 

 modern development. 



At the present time Myrmecobius is obviously in process of losing 

 rather than gaining teeth. This has been noted by Gidley who says: 

 "The variability in the molar series in Myrmecobius seems due to the 

 presence or absence of the last molar, probably a disappearing tooth. 

 In the upper jaw the last molar seems to be normally wanting, while the 

 second is apparently in the process of disappearing, being sometimes 

 present and sometimes wanting." It seems highly improbable that the 

 causes which originally brought about an increase in the tooth formula 

 are the same as those now producing a decrease. A more reasonable 



