Strophomena and Other Fossils 
65 
by the Saluda invasion, probably from the north, and occur 
only in the basal layers of the Saluda. 
With these preliminary remarks in mind, it will be possible to 
appreciate the significance of the specimens of Strophomena 
which are so abundant in the lower beds of the Richmond on 
Manitoulin. 
Perhaps the most striking feature of these Manitoulin speci- 
mens is their general resemblance to Strophomena concordensis 
and to the similar specimens associated with typical Stropho- 
mena nutans at Clarksville, Ohio, and elsewhere in Cincinnatian 
areas. The latter are described under Strophomena nutans, 
and are regarded not merely as descendants of Strophomena 
concordensis, but also as the less aberrant forms of which typical 
specimens of Strophomena nutans are the retarded, gerontic 
individuals. 
The general outline of the shell tends to be subnasute and sub- 
pentagonal. It is possible to cull out specimens in which this 
tendency is strongly marked, and to select others in which the 
convexity of the shell is more regular, and the general outline 
is about as evenly rounded as in Strophomena planumbona. 
However, shells in which the tendency toward a subnasute out- 
line is only faint, are more common. The postero-lateral angles 
usually vary between 80 and 85 degrees, but they frequently 
are rectangular, and occasionally they may be as acute as 70 
degrees. Not infrequently the shell is extended slightly at the 
hinge-line. 
The brachial valve is flattened posteriorly for a distance of 
about 10 mm., after which the downward flexure of the shell 
begins. The maximum convexity usually occurs 12 to 15 mm. 
anterior to the beak. In the subpentagonal shells, the slope 
from here to the antero-lateral margins is somewhat more rapid 
than along the median line, thus giving the central part of the 
valve a slightly humped appearance. Usually the downward 
curvature of the shell is rather regular anteriorly, but occasion- 
ally it is quite abrupt. Sometimes the nasute character of the 
shell is very pronounced and occasionally an individual is found 
in which this nasute character is as prominent as in the figure 
of Strophomena hecuba published by Billings in Paleozoic Fossils, 
vol, I, p. 126. 
