Strophomena and Other Fossils 
103 
Hinge-line usually approximately equal to or only slighter longer 
or shorter than the width of the shell across the middle, the lateral 
margin meeting the hinge-line nearly at right angles. Occasion- 
ally, the length of the hinge-line exceeds the middle width of the 
shell conspicuously, and the lateral margins of the shell may meet 
the hinge-line at angles as low as 63 degrees. 
Brachial valve flat at the umbo, usually raised anteriorly along 
the median line so as to form a more or less prominent median 
elevation. Shell usually nearly straight or only slightly convex 
from front to rear along this median elevation, especially when 
the latter is prominent. 
Pedicel valve moderately convex near the beak, more or less 
depressed anteriorly, forming a shallow median sinus along the 
anterior half of the valve. 
Radiating striae within a distance of 1 cm. from the beak 
usually between 50 and 55, but occasionally as few as 45 or 
as many as 70 in number. When the striae are numerous the 
primary striae are not conspicuously larger than the secondary. 
However, when the striae are less numerous, the former often 
are so much more conspicuous that the shell appears rather 
coarsely striated. In extreme cases the shells are striated almost 
as coarsely as in typical specimens of Strophomena sinuata. If 
de Verneuirs statement, that Strophomena sulcata occurs along 
with Strophomena planoconvexa, were taken literally, then the 
coarseness of the striations in his figures would be readily ac- 
counted for, since the radiating striations of Strophomena sinu- 
ata are typically coarser. 
Average specimens of Strophomena sulcata may be readily 
distinguished from Strophomena sinuata by the stronger median 
elevation and depression, by the straightness or moderate con- 
vexity of the brachial valve from front to rear along the median 
line, and by the less coarse or decidedly finer radiating striae. 
In shells having the more numerous and finer radiating pli- 
cations, the median elevation and depression often are but slight; 
in some cases even less than in typical specimens of Strophomena 
sinuata. When these features are accompanied by a conspicu- 
ous elongation of the shell along the hinge-line, so that the lateral 
margins form acute angles with the latter, the shells differ con- 
spicuously from the more typical quadrate forms of Strophomena 
sulcata. However, the transversely elongate forms occur only 
