60 
Aug. F. Foerste 
size or are approximately equal toward the anterior margin; or 
they may be very nearly equal in size even posteriorly. About 
25 mm. from the beak they usually vary between 12 and 16 in a 
width of 5 mm., but occasionally this number is increased to 18, 
20, and even 24. The radiating striations of the pedicel valve 
are about equal to those of the brachial valve posteriorly, but 
anteriorly they are more numerous, frequently equalling from 20 
to 24 in a width of 5 mm., although occasionally as low as 17, and 
even 14. When every fourth striation is conspicuously stronger 
than the intervening ones, their number usually equals about 6 
in a width of 5 mm. 
Aside from the larger size of the shell, the interior of the pedicel 
valve presents the most characteristic diagnostic features of this 
species. The thickened border along the anterior and lateral 
margins of the valve is never as prominent and abrupt, nor as 
well defined along the inner edge as in the case of Strophomena 
nutans. The absence of the strong vertical thickening along the 
median anterior nasute edge of the pedicel valve, and of the strong 
vascular markings on the upper and inner side of this thickening, 
is especially characteristic. Usually, the thickening along the 
anterior and lateral margins of the pedicel valve, in Strophomena 
concordensis, is only moderate; the inner margin of this thickened 
border is rather vaguely defined, and the border is crossed by 
vascular markings extending as far as the anterior edge of the 
shell. These vascular markings extend fully a centimeter from 
the anterior edge. The muscular area is deeply impressed and is 
limited by a sharp, prominent border, deflected anteriorly so as to 
produce a median gap. The muscular area is crossed by a median 
ridge, on each side of which are the impressions left by the adduc- 
tor muscles. 
In the Ohio Naturalist, vol. XII, pi. XXII, 1912, Fig. 10 A illus- 
trates a typical interior of the brachial valve. Fig. 10 B presents 
an aberrant form of the pedicel valve in which the marginal 
thickening lies nearer the edge, is more distinctly defined poste- 
riorly, and is more abrupt anteriorly than usual. 
The interior of the brachial valve does not present any distin- 
guishing features. The two-lobed cardinal process, the strongly 
divergent crural ridges, the median ridge dividing the muscular 
area anterior to the cardinal process, and the four vascular ridges 
and intermediate sinuses along the median parts of the valve are 
