FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 69 



fauna ^° of the region fairly well but far from completely. The high 

 proportion of species loiown here from only one specimen each, about 

 25 percent, shows that the collections, large as they are, are not yet 

 adequate to give a proper sample of the species that may be pre- 

 sumed actually to be available here as fossils. Such a high percent- 

 age of very rare forms indicates clearly that any further collecting 

 is almost certain to add to the number of known species. 



As far as mference can be drawn from these data and from ecologic 

 and similar considerations, the situation regarding the v>'hole pre- 

 served (but only partly collected) sample, that is, the forms that were 

 actually fossilized, is much better, but not perfectly satisfactory. It 

 may reasonably be assumed that there was in tliis general region 

 some variety among the larger and possibly plains-dw^elling types, 

 such as are common in the Torrejon and also in the later Paleocene 

 and early Eocene in acljacent areas, but their great rarity or absence 

 here and the general facies present strongly suggest that the condi- 

 tions of deposition were such that some and perhaps many of these 

 were not preserved and so will never be known in tliis area, no matter 

 how large the collections made. 



On the other hand, knowledge of the general composition of the 

 Middle and Upper Paleocene mammalian faunas of North America 

 as a whole may now be considered very good. It is probable that 

 we have representatives of almost all the orders and families and a 

 large majority of the genera,'^ that occurred on this continent during 

 that time. The combined area represented by collections is now very- 

 considerable, on the order of 1,000 square miles of actual collecting 

 territory, representing many times that in the ranges of sampled 

 faunas. The environmental variety represented is apparently great, 

 for the sediments yielding mammals of these ages are of many dif- 

 ferent sorts, many genera are represented by several well-defined 

 species in each, and the inferred habits of the various known mam- 

 mals include almost every possible terrestrial mammalian habitus. 

 The collecting areas certainly were part of a unified North American 

 land mass in the Paleocene, extend more than 1,200 miles north and 

 south, and w^ere probably central on that land mass, ideally situated 

 for a representative sample of the whole North American fauna. 



20 It would be rather hopeless to crusade against the UBiversal and careless habit of calling a collection a 

 fauna. By "real fauna" is meant what should properly be called simply"fauna"; that is, the totality of 

 mammals that actually lived in this area, and not merely those that happen to be known or the collection, 

 a sample, on which this knowledge is based. 



2' In accordance with the tentative views as to adequacy of local samples expressed elsewhere (Simpson, 

 1936a). 



