NO. I LOWER EOCENE MAMMALIAN FAUNAS — GAZIN 79 



than in the type, but the correspondence is closer than it is to the type 

 oi H. cdciculus. 



With regard to the New Fork materials of Heptodon, excluding a 

 very small Dp*, the average size of teeth in some 20 specimens is 

 distinctly larger, including specimens more nearly corresponding to 

 the type of H. posticus. There is, nevertheless, an overlap in range 

 with the La Barge suite. These New Fork materials are surely of a 

 single species but, because of overlapping range, they should probably 

 be regarded as representing a mutation in time. Possibly this could 

 be cited as H. ventorum mut. posticus. 



A similar situation, but with specimens of smaller average size than 

 in the La Barge sample, is noted in the Fossil Basin collections con- 

 sidered as Lysitean in age. Material from the type Knight and from 

 the distinctly reddish beds high beneath the Green River shale on 

 Fossil Butte include the specimens cited in 1952 as Heptodon ven- 

 torum. Additional material from both localities, including an M., from 

 Knight station and four lots of associated tooth fragments from 

 Fossil Butte, corresponds more nearly to the H. calciculus type. Again 

 the degree of difference involved here cannot be regarded as more than 

 subspecific and represents a mutation in time. Nevertheless, in con- 

 sidering the upper and lower ranges represented, for example, by 

 H. ventorum calciculus and H. ventorum posticus, a difference is 

 involved that is surely of full species value. It appears, moreover, that 

 we are faced with an association of these two in the New Fork horizon 

 from which a single very small Dp* can scarcely be included with the 

 material oi H. v. posticus. A small tapiroid seems evident also in the 

 Wasatchian(?) beds to the northwest of the Oregon Buttes, repre- 

 sented by a comparatively small jaw in which the molar teeth have 

 lost most of their enamel. I suspect that Bridger Helaletes was 

 derived from a small variant of Heptodon and not H. v. posticus. 



A situation particularly difficult of interpretation is presented by 

 the Heptodon materials from the Dad locality. The four specimens 

 from here in the National Museum collections, known to have come 

 from the same horizon, can readily be allocated to Heptodon ven- 

 torum (s.s.), much as in the case of the La Barge materials. One of 

 these (U.S.N.M. No. 22498; pi. 13, fig. 12) is a well-preserved lower 

 jaw which corresponds rather closely in length of tooth row to the 

 type of H. ventorum, although the width of the teeth is a little less. 

 Included in the University of California materials from the Dad area, 

 however, are remains of an immature individual (No. 43703) which 

 includes maxillae, jaws, and some fragmentary limb material of a 



