210 



BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



The upper teeth, although much smaller and lighter in construction, 

 resemble D. protenus rather than D. haydenianus. These are probably 

 primitive characters, and D. haydenianus is slightly aberrant. The 

 development of P3_4 is also somewhat closer to D. protenus, and it is 

 not clear that this is wholly in primitive characters. The presence 

 of only one talonid cusp on P4 may be, but it is unlikely that the more 

 cuspidate P3 is primitive. The development of relatively large and 

 shearing anterior cuspules on these teeth is different from either 

 D. haydenianus or D. protenus and seems to be a specialization. 

 M2 is perhaps merely primitive v/ith respect to either of the other 

 two species. 



It is possible that D. microlestes stands nearer the D. protenus 

 ancestry than does D. haydenianus, in which case the subgeneric 

 separation might well be maintained, and D. microlestes would belong 

 to the typical subgenus. The evidence for this is not very good, 

 however, and for the present it seems best not to attempt a subdivision 

 of the genus. 



There is another Didymidis-like species in the fauna, D. tenuis, of 

 very diminutive size. At present it is known from a single specimen, 

 and, as noted below, there is some doubt as to the chare ters of P4. 

 If they were confirmed the species could hardly be placed in Didy- 

 midis, but knowledge is now so imperfect that it seems best to leave 

 it here until further evidence is at hand. Table 47 gives an idea of 

 the very considerable differences in the length of Mi among the three 

 Middle Paleocene species referred to Didymidis: 



Table 47. — Length 0/ Mi in three species of Didymictis 



There are also a few specimens too imperfect for exact identification 

 that are comparable in size to D. haydenianus and cannot at present 

 be distinguished from that species. 



DIDYMICTIS MICROLESTES Simpson 



Figures 56, 57 



Didymictis microlestes Simpson, 1935d, p. 238. 



Type.— U.S. l^M. no. 9301, left lower jaw with P4-M2. Collected 

 by Dr. J. W. Gidley. 



