70 
Aug, F. Foerste 
the valve. The ovarian pits usually are conspicuous only on 
the posterior part of the pedicel valve. Occasionally the shell 
is slightly extended at the hinge-line. The brachial valve, of 
course, presents a similar outline. The impression of the adduc- 
tor muscles usually is deep, and strongly defined by the callosity 
bordering the anterior part of the crural plates and the cardinal 
process. There is a strong medial elevation, separating the ad- 
ductor scars, anterior to which are the median ridges separating 
the vascular grooves. 
While it is evident that the smaller specimens, described in 
the preceding lines, are to be regarded as the types of Stropho- 
mena nutans^ it must be remembered that they are gerontic indi- 
viduals secured by selection from a much larger series, in which 
the callosity along the interior border of the pedicel valve is 
much less conspicuous, and which frequently attain a much larger 
size. Some of these larger specimens, occurring in the same 
slabs of rock, for instance in the Rocky Hollow northwest of 
Clarksville, Ohio, attain a width of 32 mm. a length of 25 mm., 
with a convexity of 14 mm. In all of these larger specimens, the 
thickening along the interior border of the pedicel valve is much 
less conspicuous, not exceeding a height of 5 mm. above the 
plane passing through the margins of the valve, or a height of 
6 mm., measured along the slope of this border. Moreover, this 
thickened border is less distinctly limited on the posterior side, 
and the transverse radial vascular grooves extend farther toward 
the muscular area. In still other shells, from the same layers, 
the thickening along the border becomes slight, and the shallow, 
radial vascular grooves are inconspicuous. Shells of this type 
closely resemble Strophomena concordensis , from the top of the 
Arnheim, and some individuals could scarcely be distinguished 
from the latter if their origin were not known. In fact, the 
smaller, typical specimens of Strophomena nutans are regarded as 
the retarded, gerontic forms of Strophomena concordensis. Fig. 
1 d, accompanying the original description of this species by 
Meek, may represent the interior of the brachial valve of one of 
these larger forms associated with the smaller, narrower, and 
more typical forms of Strophomena nutans. (Plate X, Fig. 3.) 
Where abundant material is at hand, there is no difficulty in 
distinguishing between Strophomena nutans and Strophomena con- 
