Strophomena and Other Fossils 
61 
distinctly shown. Of these ridges, the two lateral ones terminate 
posteriorly within the impressions of the adductor muscles. Pos- 
teriorly, the interiors of both valves occasionally are pitted. Aside 
from the markings already mentioned, numerous minute papillae 
often may be seen under a lens, the surface appearing smooth to 
the unaided eye. 
From Strophomena wisconsinensis this species may be distin- 
guished by its more triangular, subnasute outline, and the finer 
striation of the exterior surface. The height of the hinge-area 
also is much less. 
The type specimens of Strophomena concordensis were found in 
a partially indurated blue clay layer at the top of the Arnheim 
bed, a short distance south of the railroad bridge, east of Concord, 
Kentucky, at creek level. 
Strophomena concordensis ranges from the southwestern corner 
of Butler county, in Ohio, southeastward to Maysville and Con- 
cord, along the northern edge of Kentucky. In all of this dis- 
tance its horizon is confined to the top of the Arnheim, usually in 
a chunky, bluish, argillaceous rock, although in the southern part 
of Adams county this gives way to more or less limestone, and 
southeast of Maysville, in Kentucky, the Strophomena concorden- 
sis horizon consists practically entirely of limestone layers. East- 
ward, the chunky, argillaceous rock varies from 5 to 7 feet in 
thickness, but north of Lebanon, Ohio, its thickness is 4} feet; 
opposite the Chautauqua grounds at the southern edge of Mont- 
gomery county, it is 3 feet; and westward it is still thinner, dis- 
appearing before reaching the western boundary of Butler county. 
At Oregonia, Strophomena concordensis is very rare and occurs 
only at the base of the chunky argillaceous rock. This locality, 
and the one south of Lebanon, appear to mark the northern 
boundary of the area of distribution for this fossil. At many of 
the localities farther southward, Strophomena concordensis occurs 
in great abundance throughout the entire thickness of the chunky 
argillaceous rock, at the top of the Arnheim. The interval 
between this rock and the underlying Dinorthis carleyi horizon 
usually varies from 16 to 20 feet. 
Among the areas at which Strophomena concordensis does not 
occur, notwithstanding the presence of the chunky, blue, argil- 
laceous rock at the top of the Arnheim, is the mouth of Lick Bun, 
opposite the mouth of Caesar Creek, in the northern part of War- 
