Strophomena and Other Fossils 
59 
specimens in Kentucky. The species, owing to its associations, is 
regarded as of southern origin. This is to be expected, if it is a 
derivation from Strophomena maysvillensis. 
Strophomena concordensis^ Foerste^ 
{Plate III, Figs. 1 A, B, C, D, E, F, G, H, I, J, K, L, M, N) 
Strophomena concordensis evidently is closely related to Stro- 
phomena nutans, but it is a much larger species than the typical 
form of the latter. Specimens frequently attain a width of 42 
mm., a length of 30 mm., and a convexity varying from 11 to 
15 mm. The convexity of the more strongly arched specimens 
varies between 0.44 and 0.48, although frequently less. In Stro- 
phomena wisconsinensis this ratio may equal 0.55, or even 0.60. 
The lateral outlines, posterior to the middle of 'the shell, frequently 
are subparallel or only moderately divergent, as in Strophomena 
nutans, but more frequently the outline of the shell is sub triangu- 
lar, the postero-lateral angles varying from 75 to 55 degrees. In 
Strophomena nutans, the outlines more commonly are subpen- 
tagonal, and shells having a subtriangular outline are far less 
common. The subtriangular appearance of the brachial valve 
is strengthened by the downward deflection of the antero-lateral 
parts of the valve. The posterior part, within 10 mm. of the 
hingedine, usually is distinctly flattened, but the concavity of 
the valve immediately anterior to the beak is almost imper- 
ceptible. The convexity of the umbonal parts of the pedicel 
valve, immediately anterior to the beak, is moderate, but is much 
more readily perceptible than the corresponding almost imper- 
ceptible umbonal concavity of the brachial valve. The concavity 
of the middle part of the pedicel valve varies, of course, with the 
general convexity of the brachial valve. In extreme cases it is 
deeply concave, the maximum concavity being at or immediately 
anterior to the middle of the shell. Oblique wrinkling of the shell 
along the hinge-line may be detected occasionally. 
The radiating striae of the brachial valve are very fine near the 
beak, but become coarser toward the anterior margin of the shell. 
Frequently every third, fourth, or sixth striation is more promi- 
nent, especially anteriorly, but in some specimens they alternate in 
^ Denison Univ. Bull,, vol. xiv, p. 213, 1909. 
