74 
Aug, F, Foerste 
detected, the anterior parts of the shell appear ornamented by 
rather distant stronger striae of which 8 or 9 may appear in a 
width of 5 mm. 
On the brachial valve represented by Fig. 4 the striae along 
the anterior margin are more nearly subequal, owing to the increase 
in size of the intermediate finer striae anteriorly, and here about 
15 or 16 striae are noted in a width of 5 mm. 
In the specimen represented by Fig. 4 a, the length is 20 mm. ; 
the width, 25 mm.; and the convexity, about 6 mm. In the 
specimen represented by Fig. 4 h, this convexity equals 7 mm., 
and in the most gerontic specimens it may attain even 11 mm. 
The shells have a distinctly quadrate outline, a very slight con- 
traction in width immediately anterior to the hinge-line being 
balanced by a slight elongation at the hinge-line itself. The 
brachial valve is distinctly flattened for a distance varying from 
12 to 14 mm. anterior to the hinge-line. The concavity imme- 
diately anterior to the beak is slight. The hinge-area is rather 
high, considering the width of the shell, and diminishes rather 
gradually until near the postero-lateral angles of the shell. In 
specimen 4 a, the height of the hinge-area is 3.3 mm. at the beak; 
in specimen 4 5, it is 3 mm. 
In general, the shells of Strophomena planumbona are small 
or attain only medium size. The posterior third of the brachial 
valve is strongly flattened. Surface striae, fine and thread-like, 
about 13 in a width of 5 mm.; occasionally as few as 11, rarely 
as many as 20. Muscular scars of the pedicel valve, producing 
a circular, deeply impressed area, bordered on each side by a 
curved, sharply elevated ridge, deflected slightly anteriorly, pro- 
ducing an anterior median gap. Median vascular markings 
often distinct. The interiors of the pedicel valves are thickened 
toward the anterior and lateral margins, this thickening often 
being rather distinctly defined posteriorly, and crossed by branch- 
ing vascular grooves. Along the sides of the pedicel valves, this 
interior thickening becomes narrower, though still fairly dis- 
tinct and showing vascular markings. 
In Stony Hollow, northwest of Clarksville, in Clinton county, 
Ohio, Strophomena planumbona makes its first appearance 1 
foot above the 5-foot clay layer in which Orthoceras fosteri is 
common. It therefore begins at the base of the Clarksville 
division, and from this level it continues through the Clarksville 
