36 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL. XXXIV. 
the species cited. They have been taken with their accepted mean- 
ings. This method is sufficiently accurate for practical purposes 
so long as the chief object in view is to identify the faunal agere- 
gates and to define and locate them stratigraphically and geograph- 
ically. | 
This general work has already been accomplished for a limited 
area and for the faunas ranging through a limited portion of the 
geological time scale. It 1s now proposed to take a step in advance 
and to attempt to discriminate the species of a single genus for the 
field under investigation on the basis of their geologic range and 
geographic distribution. 
In any specific definition it is, of course, important to take note 
of characters by which the specimens described are distinguished 
from other representatives of the genus; but to distinguish species 
alone by the conspicuousness of their characters tends to emphasize 
the aberrant and rare. The dominant rather than the rare, the 
common rather than the aberrant characters must be used in dis- 
criminating the characteristics of natural species. But in seeking 
the dominant characteristics of a species the basis upon which the 
ageregation of the individuals making up the group is founded 
must also be natural and not artificial. In some cases the specimens 
selected by authors as types of their specific definitions are fairly 
good representatives of the natural species to which they belong, 
but in numerous cases of paleontologic species it 1s discovered, after 
the definitions have become established, that the selected types are 
actually aberrant forms, rare and not characteristic of any natural 
ageregation of individuals. 
Without positively knowing the cause of aggregation it 1s possi- 
ble to distinguish natural from artificial aggregates. Examples of 
natural aggregates of organic individuals are (a) the contents of a 
single stratum at a single outcrop (that is, a. faunule); (>) the 
series of successive faunules in a single section through which range 
the majority of the same species holding approximately the same 
proportions of abundance in comparison with other species (that is, 
a fauna considered geologically); and (¢) the group of faunules 
spread over a particular geographic area and extending to the limits 
within which the particular species occupies the same general place 
of abundance or rarity in the composite fauna (that is, the fauna 
geographically considered); all these are natural aggregates. On 
the other hand, the specimens in a museum labelled Hamilton or 
Trenton or Cretaceous do not necessarily constitute natural aggre- 
gates for purposes of specific description. It is also probable that 
no set of specimens combined on the basis of exhibiting the same 
characters is necessarily on that account a natural ageregate of 
individuals. 
