Strophomena and Other Fossils 
29 
Postero-lateral angles varying from 70 to 85 degrees, giving rise 
to two types of outline, in one of which the postero-lateral angles 
vary from 70 to 75 degrees, and the shell is broadly and trans- 
versely semi-elliptical. In the other type of shell, the postero- 
lateral angles equal about 85 degrees, and the outline is subquad- 
ratic. Apparently the shells with the more acute postero-lateral 
angles are less convex, while the convexity of the more quadratic 
forms is greater. This convexity usually equals 4 or 5 mm. in the 
former group and 6 or 7 mm. in the latter. The brachial valves are 
evenly convex from front to rear, with a scarcely perceptible 
depression about a millimeter from the beak. The concavity of 
the pedicel valve varies from 1 to 2 mm. Oblique wrinkling of 
the shell along the hinge-line is rare, and not at all characteristic 
of the species. In one specimen these wrinkles formed an angle 
of about 40 degrees with the hinge-line. The number of radiating 
striae usually equals 6 or 7 in a width of 2 mm., but may be as low 
as 5, and as high as 9 or 10, in the case of individual specimens. 
Deltidium of the pedicel valve broadly convex, the teeth extend- 
ing only a short distance, usually less than a millimeter, beyond 
the hinge-area. Muscular area rounded, subrhomboidal, with a 
strongly raised lateral border posteriorly, becoming less conspicu- 
ous anteriorly, where it is deflected toward the front, leaving a 
gap varying from 2 to 3 mm. in width. The muscular area is 
traversed by a low, broad, median elevation, bearing indications 
of the attachment of the adductor muscles posteriorly. There 
is a tendency toward a linear depression on each side of this me- 
dian elevation, extending forward through the gap. The area of 
attachment of the diductor muscles frequently is divided by a 
miore or less distinct ridge into two parts, of which the one nearest 
the postero-lateral border is the narrowest. There are no radiat- 
ing striae of about equal size and even distribution as in the case 
of Strophomena incurvata. The type of muscular area seen in 
Strophomena vicina is that typified by Strophomena planumhona, 
of which it may be regarded as a precursor. The interior of the 
body cavity is comparatively smooth. There is a slight thicken- 
ing of the valve about 4 mm. from the anterior margin, this 
thickening usually equalling less than a millimeter. It is crossed 
by vascular grooves, branching anteriorly and merging along the 
margin into the numerous grooves corresponding, in position, to 
the radiating striae on the exterior of the shell. 
