williams.] COMPARISON WITH NEW YORK SPECIES. 71 
showing the variational tendency to diminish the number of plications, 
which is a conspicuous feature of the dominant representatives of the 
race when looked at in geologic succession. 
The second specimen is a variety of B. pyvamidata^ which typically 
exhibits few plications as soon as they are evident in the beak region. 
This variant is much narrower in form than the typical specimens, and 
exhibits but a single plication in the fold instead of three, as is usual, 
and only four on each side, the typical number being five. In this 
case the central plication is longer and more prominent and more 
elevated from the surface than is usual. If this were represented by 
many individuals holding these characters without variation, it would 
undoubtedly be considered as a good species; but if we regard the 
rhynchonellas of this group as particularly plastic in these elements of 
form, the specimen is to be regarded as a marked variational form 
of the B. jjyramidata type, carrying its peculiarities of B. pyramidata 
a step further away from the central form in the direction of the varia- 
tion which the normal pyramidata has assumed. Both of these 
aberrant forms are evidence of the nature of the plasticity among the 
individuals living together at the time of the Square Lake limestone. 
When it is observed that the same characters are those upon which 
dominant groups of individuals, called species, are founded, not only in 
this fauna, but in the several New York faunas of the Lower Helder- 
berg, it becomes evident that what constitutes the limitation of a 
species is the checking of the plasticity of the several characters at the 
same point in numerous individuals. 
This characteristic of specific groups is brought out by comparing 
species of successive faunas. In a single fauna, like this Square Lake 
fauna, it is possible to see only the combination of characters marking 
the dominant forms and those divergent forms associated with them 
which, because of their rarity, are clased as varieties. In a series of 
successive faunas it is also possible to see the changes in combination 
of characters marking the successive species, each successive species 
bearing the relation of mutant to the earlier forms, but the dominant 
combination is not a direct mutant of the earlier dominant forms, but 
may be a mutant of some of the inconspicuous varieties. 
In the New York Lower Helderberg we find a convenient series 
upon which to make these observations. The rhynchonellas of this 
general group are described by Hall under the following names: 2 For 
the Lower Pentamerus fauna, B. mutabilis Hall; for the Delthyris Shaly 
limestone, B. nucleolata, B. abrupta, and B. vellicata Hall; for the 
Upper Pentamerus, B. ventricosa, B. nobilis, and B. canupbellana; for 
the Oriskany, B. speciosa; and other species may be added, but their 
divergence is so extreme as to bring in other questions which may be 
1 Not figured on these plates. 2 See Pal. New York, Vol, III. 
