26 
Aug. F. Foerste 
be quite inconspicuous. This thickened region is crossed radially 
by rather distant grooves which may be traced posteriorly for a 
short distance, sometimes about half the distance from the thick- 
ened region to the muscular area. Toward the margin of the 
valve, these grooves frequently branch several times. 
Excepting within the region of the muscular area, the interior 
of the brachial valve is minutely granulose, with indications of 
short radial ridges corresponding in position to those already 
described in case of the pedicel valve. A low elevation extends 
from front to rear across the muscular area. Two low but rather 
narrow^ and sharp ridges, 6 to 7 mm. long, and 5 mm. apart, extend 
across the middle of the valve, in a direction from front to rear. 
These ridges correspond to the outer pair of the four ridges limit- 
ing the median vascular sinuses in such species as Strophomena 
vetusta. The anterior and posterior adductor scars are rather 
indistinctly defined. The cardinal process is bifid, the two lobes 
being divergent; the posterior faces of these lobes are crossed by 
a groove. The crural plates are strongly divergent, and become 
subparallel to the hinge-area within mm. of the latter. 
Strophomena incurvata was described by Shepard from the vicin- 
ity of Green Bay, Wisconsin, where it occurs in strata regarded 
as equivalent to the Black River formations. 
In the lower part of the equivalent strata of Minnesota, Stro- 
phomena incurvata is associated with Rafinesquina minnesotensis, 
Orthis tricenaria, Dinorthis deflecta, Hehertella hellarugosa, Dal- 
manella suhaequata with several of its varieties, and Rhynchotrema 
inaequivalve. All of these species, with the exception of Dinor- 
this deflecta, are represented also in the upper part of the Black 
River section of Minnesota, either by typical specimens or by 
varieties of the species. Dinorthis pectinella is listed only from 
the upper part of the Black River in Minnesota. 
The association of Strophomena incurvata with Orthis tricenaria, 
Dinorthis pectinella, and Hehertella hellarugosa is found on Cloche 
Island, northeast of Little Current, west of Georgian Bay, in 
Ontario. South of Cloche Island, on the northeastern edge of 
Goat Island, Strophomena incurvata is associated with Orthis 
tricenaria, Dinorthis pectinella, and species of Carahocrinus and 
Cleiocrinus. These are typical Black River exposures. 
In central Kentucky, the large typical specimens of Orthis 
tricenaria and Dinorthis pectinella, associated with Carahocrinus 
