ADDENDA. 
415 
Centre, Medina county, Ohio, in rocks pertaining to the Waverly group. 
The specimens present no characteristic differences from those of the Lower 
Helderberg group, or any modifications which cannot be detected among 
the collections of specimens from that group. Individuals may be selected 
from any of the formations that will show strong and well marked differ¬ 
ences, either from these under consideration, or from each other, but a 
series connecting the extremes from either the Niagara or Lower Helderberg 
groups will contain individuals having the same characters with those pre¬ 
sented by the Ohio specimens. 
The occurrence of this species in this Geological position, is the more 
interesting since thus far, it has not been found in the Hamilton or Che¬ 
mung groups of New York. 
Strophodonta subdemissa. 
PLATE XVII, FIGS. 2 h, 2 i. 
Strophodonta subdemissa: Hall, Tenth Report on the State Cabinet, p. 145. 
Strophodonta demissa: Hall, Geological Report of Iowa, Vol. I, part 2, p. 495, pi. 5, a, b, c. d 
Strophodonta dimosa ? [demissa] Owen. Geol. Survey of Wisconsin, Iowa, Minnesota. Explana¬ 
tion of Table III a, fig. 14. 
Shell semicircular in outline, with more or less extended and sometimes 
mucronate cardinal extremities. 
Ventral valve depressed convex, somewhat regularly arching from beak to 
base ; area narrow, crenulated. 
Dorsal valve concave, nearly following the curvature of the ventral valve; 
area linear, crenulated. 
Surface marked by strong radiating striae, which are increased by intersti¬ 
tial additions becoming finer and more numerous near the margin in adult 
individuals. The striae are themselves longitudinally striate on well pre¬ 
served specimens, having from three to six striae on each of the larger 
ones, near the middle portions of the ventral valve. 
I have hesitated to continue this as a distinct species, and have so indicated on 
page 114 of this volume. The Western specimens, however, show a constant dif¬ 
ference in form and in certain peculiarities of the striae. While the New York 
