74 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
count of the plications as figured for R. speciosa and R. rnultistriata, 
we find the following: 
R. speciosa, twenty-seven to thirty -three plications, not bifur- 
cating on body of shell. 
R. barrandi, thirt}^ to forty plications, rarely bifurcating. 
R. ramsayi, thirty-six to forty plications. 
R. pliopleicra, sixty-seven to seventy plications, bifurcating. 
R. fitcJiana, seventy-five plications, bifurcating. 
R. principalis, eighty plications, occasionally bifurcating. 
R. oblata, seventy-five to eighty plications, occasionally bifur- 
cating. 
R. iiiultistriata, seventy-five to eighty plications, bifurcating, 
finely striate. 
Before a careful study of these facts (upon actual series of specimens) 
is made it would be unscientific to attempt to state the actual limits 
of specific variation; but it is evident that the Oriskany species are 
as a series more numerously plicated; the fold and sinus, particularly, 
hold more plications; the size of the specimens is greater and the 
bifurcation of the plications is a common characteristic, causing them 
to differ from the series of forms which, on the theory of descent, 
must be supposed to contain their ancestors. 
Without, at the present time, turning aside to investigate the actual 
facts regarding the New York Helderberg series, it is evident that the 
morphologic mutation which the representatives of this group of 
rhynchonellas expressed included the following particulars: 
The variations which were expressed by the individuals of the Lower 
Pentamerus single species R. mutabilis were carried to the extent of 
making greater divergence of form in the Delthyris Shaly limestone 
stage. In the Upper Pentamerus the extremes were still greater, but 
the variation was less, and the dominance in number of individuals is 
among the extreme and not the median types. 
In the Oriskany the characteristic mutation resulted in great size 
of individuals, associated with which was a decided increase in the 
number of plications, chiefly by bifurcation. This last fact, if taken 
in connection with the normal variability or, more strictly speaking, 
plasticity in the number of plications, suggests the inference that the 
increment in the number of plications is associated with expansion of 
the surface of the shell laterally. In case this took place very early 
in development, bifurcation resulted with growth. But when vigorous 
growth took place' in the middle and later stage of individual develop- 
ment, the resultant effect was gibbosity of form. When, however, 
vigorous growth began early and continued throughout development, 
the bifurcation of the ribs continued to take place and great size was 
attained, as in the Oriskany forms. 
