116 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



mentators, the evidence has not been clearly stated in publication and 

 Gidley's abandoned view is becoming entrenched in the literature. 

 It is therefore necessary to consider it here. 



Gidley did not explicitlj^ discuss the evidence for referring Myrmeco- 

 boides to the Marsupialia. It is clear that the reference was based 

 almost entirely on the molariform character of the fourth cheek tooth, 

 and in part simply on a general resemblance to the one marsupial genus 

 Myrmecohius. Gidley mentions the three possibilities regarding the 

 molariform tooth: that it is a true molar, that it is a deciduous molar, 

 and that it is a true premolar. He cites the narrower crown, smaller 

 talonid, and large, anteriorly directed paraconid as seemingly preclud- 

 ing the possibility that the tooth is a true molar. This evidence is 

 valid, and to it may be added (as first pointed out by Matthew) the fact 

 that the protrusion and wear of the tooth definitely prove that it was 

 erupted long after the following tooth, which could not be true of a 

 first molar. Gidley states that the very complex structure in com- 

 parison with P3 opposes the supposition that this is P4 and supports 

 his conclusion that it is dm4, retained in the adult dentition. But the 

 relative time of eruption, as first shown by Matthew, is impossible for 

 dm4, which in all known mammals is erupted before Mi rather than 

 long after, as the tooth in question is in Myrmecoboides. Further- 

 more, the fourth cheek tooth in marsupials, whether it be Mi or dm4, is 

 erupted before the following tooth or at almost the same time. Gidley 

 also seems to have overlooked, momentarily, the fact well known to 

 him that in the Leptictidae and some other unquestionable placentals 

 P4 is characteristically as molariform as this tooth, or more so, even 

 though P3 may be as simple as in Alyrmecoboides. It may be con- 

 cluded, and Gidley also did conclude on more mature deliberation, 

 that this is certainly P4, and hence that its evidence is entirely opposed 

 to marsupial and in favor of placental affinities for the genus. 



Gidley analyzed all the resemblances to and differences from 

 Myrmecobius. The only resemblances that could be supposed to 

 carry much weight are the spacing of the premolars and the elevation 

 of the internal cusps of the molars. These characters also do occur in 

 placentals and are highly aberrant among marsupials. These, and all 

 the other lesser points of resemblance, would perhaps tend to link 

 Myrmecoboides to Myrmecobius if the former were otherwise proved to 

 be a marsupial, but they have no value as to the primary question of 

 showing it to be marsupial. There are indeed profound differences 

 between the fossil and Myrmecobius. Gidley recognized these but con- 

 cluded that degeneration could lead to this great transformation, which 

 is true but is not evidence that it did so. On the other hand, Myrmeco- 

 boides has the basic and diagnostic characters of the Leptictidae, and the 

 Myrmecobius-\\]s.e characters can only be interpreted as superficial, aber- 

 rant, habitus characters bringing about a slight degree of convergence. 



