Strophomena and Other Fossils 47 
Strophomena maysvillensis, Foerste^ 
(Plate IT, Figs. 4 A, B, C, D, E, F, G, H, I, J) 
Strophomena maysvillensis is closely related to Strophomena 
planoconvexa. On the average, however, it is a larger and longer 
species, and the outline frequently is subtriangular, owing to the 
downward deflection of the antero-lateral parts of the brachial 
valve. The sides of the shell frequently converge posteriorly, 
the hinge-line being shorter than the width of the shell across the 
middle. Sometimes the sides meet the hinge-line at right angles, 
and occasionally even at the slightly acute angle of about 80 
degrees. The pedicel valve is only moderately concave toward 
the middle, or even is almost flat, but the brachial valve frequently 
is rather strongly convex, especially in the more triangular speci- 
mens. In the broader, less triangular forms, the convexity usu- 
ally is not conspicuously greater than that of typical specimens 
of Strophomena planoconvexa. The flattening of the brachial 
valve anterior to the beak is nearly obsolete. The ratio of the 
length of the shell to the length of the hinge-line averages between 
0.84 and 0.87, but may be as low as 0 70, or as high as 1.00; the 
ratio of the convexity of the shell to the length frequently equals 
0.40, but shells of less convexity are commmn. Hinge-area high, 
forming an angle of about 45 degrees with the plane of the pedicel 
valve. 
The deeply impressed muscular area of the pedicel valve is 
nearly circular in form, bordered on each side by a strongly ele- 
vated, sharp, curved ridge, deflected slightly forward at the gap 
at the anterior margin of the area. The interior of the pedicel 
valve is thickened a short distance within the anterior and lateral 
margins of the shell. The marginal part of the valve, exterior 
to the thickened part, frequently has a width of 2 mm. anteriorly, 
the distance across the thickened area being 4 or 5 mm. at the 
median parts of the shell. This thickened area or border is crossed 
by vascular grooves and ridges, and the median part is connected 
with the anterior gap of the muscular area by rather indistinct 
median grooves and ridges. 
The posterior outlines of the adductor areas of the brachial 
valve are distinctly defined by the callosity beneath the cardinal 
^ Denison Univ. Bull., voL xiv, p. 212, 1909. 
