no 
Aug. F. Foerste 
extensive, and if this be a constant specific difference, it warrants 
the separation of Strophomena planodorsata from that species. 
It is doubtful, however, whether the extensive flattening of 
the posterior part of the brachial valve in the type of Stropho- 
mena planodorsata will prove even a fairly constant specific 
feature. A number of large specimens, apparently identical 
with typical Strophomena planodorsata, occur in the T. E. Sav- 
age collection from the Fern vale limestone at Thebes, Illinois, 
but the specimens are all pedicel valves. These valves have 
about the same curvature as that indicated for the pedicel valve 
of the type in Fig. 6 A on plate IX. The valves vary in length 
from 35 to 40 mm., the reversal of the curvature begins at 20 
mm., but becomes distinct at 25 mm. from the beak. The 
number of radiating striae varies from 7 to 8 in a width of 2 
mm., where not increased by implantation of intermediate striae. 
It is probable, however, that the brachial valve has a much more 
pronounced curvature and is ornamented by distinctly coarser 
striae than those on the pedicel valve. 
Various specimens, from the Richmond group at Wilmington, 
Illinois, have been identified by Schuchert with Strophomena 
planodorsata. Some of these present views of the interior of the 
pedicel valve. One specimen was used for Fig. 10, on plate 
XXXI, of the Alinnesota Geological Survey, vol. Ill, part 1, and 
the same specimen, belonging to the Schuchert collection, is 
illustrated by Fig. 6 on plate VII of the present Bulletin. The 
height of the hinge-area in this specimen is 4.2 mm. The mus- 
cular area is nearly circular, and is characterized by flabellate 
markings separated by a single distinct median ridge. The inte- 
rior of this area rises gradually toward the limiting ridge, which 
descends abruptly into the body cavity. Posteriorly it approaches 
the hinge-line exterior to the teeth. The curvature of this valve 
is indicated by Fig. 7 on plate IX. 
Similar interiors of pedicel valves of equal size, from the Fern- 
vale limestone at Wilmington, Illinois, collected by Prof. T. E. 
Savage, indicate that the narrow thickened border, along the 
anterior and lateral margin of the pedicel valve figured by 
Schuchert, is not a constant feature, and usually is absent. In- 
stead, there is a moderate thickening of the interior, with its 
maximum between 5 and 7 mm. from the margin, fading away 
posteriorly. This thickened area is crossed b}^ rather incon- 
