66 
Aug, F. Foerste 
The pedicel valve is rather strongly concave, especially in the 
more nasute specimens, frequently equalling 3 and 4 mm. in 
shells 22 to 30 mm. in length. Immediately anterior to the 
beak, the valve is distinctly convex, the reversal of curvature 
taking place between 12 and 15 mm. from the beak. 
So many of the shells are obliquely wrinkled along the hinge- 
line that this feature may be said to be characteristic of the 
Manitoulin species, notwithstanding the numerous individuals 
in which this wrinkling is inconspicuous or absent. The angle 
made between the wrinkles and the hinge-line usually varies 
between 30 and 40 degrees. Radiating striae subequal or alter- 
nately larger and smaller, about 12 or 13 in a width of 5 mm., 
but ranging occasionally from as low as 10 to as high as 14 or 15. 
Individual specimens are found in which the alternation of single, 
stronger striae with three finer ones occurs, but the stronger 
striae rarely are prominent, and this type of striation is not 
characteristic of the species. 
Interior of the pedicel valve very much as in Strophomena con- 
cordensis. The thickening along the border of the shell in the 
majority of specimens is low, and is crossed by shallow vascular 
grooves, which extend halfway from the anterior margin toward 
the muscular area, in some individuals almost reaching the latter. 
In other specimens, this thickening is greater and is more dis- 
tinctly limited posteriorly, but its height rarely exceeds 2 mm., 
although in one specimen it equalled 3 mm. Only a single speci- 
men (Fig. 8, plate XI) was found in which this thickening was 
as prominent and knotted’^ along the median line as in the typi- 
cal forms of Strophomena nutans. This smaller, gerontic individual 
may owe its form to the same conditions as those which gave 
rise to the numerous specimens which are included under typical 
Strophomena nutans, in Ohio and Indiana. 
Interior of the brachial valve also essentially as in Stropho- 
mena concordensis. Crural plates making an angle of 50 degrees 
with the hinge-line. Posterior part of the adductor muscle scars 
sharply limited by the callosity which is anterior to the crural 
ridges and which unites with the median ridge separating the 
adductor areas. In most specimens, the subparallel ridges 
traversing the median parts of the valve anterior to the adductor 
areas, and limiting the vascular grooves, are not distinctly defined 
except within a short distance of these areas. 
