Foerste.] 
306 
May 1, 
a young ventral valve, Fig. 37 a dorsal valve, and Fig. 36 one of 
the intermediate stages of the ventral valve. If those forms at- 
tained to a large size without becoming doubly curved, we are sure 
we should not know how to distinguish them from var. pluri- striata. 
Strophomena obscura, Hall (?). 
(PLATE VI, FIGS. 15, 16.) 
Specimens of this species are common at Cumberland Gap, 
Tenn. Shell broader than long, the cardinal margin exceeding the 
width of the shell ; the sides forming angles of about eighty de- 
grees with the cardinal margin, near this margin perhaps a little 
produced, towards the anterior margin rounding into the semicir- 
cular curve of the same. 
Dorsal valve moderately convex, greatest convexity near the 
beak, thence sloping gradually towards the anterior margin and the 
sides, forming a more or less flattened surface between the beak 
and the anterior margin which is usually most marked in the an- 
terior half of the shell. The cast of the valve shows only two 
short depressions at the cardinal margin forming angles of about 
eighty or ninety degrees with one another and in the specimens at 
hand not sufficient to determine their dorsal character. 
Ventral valve moderately, but quite evenly convex, the highest 
part of the valve lying between the posterior third of the shell and 
the middle, thence sloping quite regularly to all parts of the valve, 
except perhaps near the postero-lateral portion of the shell which 
is slightly flattened or evenly very moderately concave ; two short 
impressions at the cardinal margin, forming angles of about one 
hundred degrees with one another and a faint mesial depression 
fails to distinguish this definitely as the ventral valve. 
The surface of both valves are covered with radiating striae, 
which are quite sharp and prominent in the casts of the valves ; of 
these there are usually five in a width of two millimetres ; between 
these there are more or less regularly appearing smaller striae, also 
much less prominent, so that the number of striae is thus usually in- 
creased to seven in two millimetres. When these smaller striae occur 
with any regularity the small and large striae alternate, but between 
two larger striae, the smaller often fails to appear. 
In the following table the last column gives the number of only 
the larger striae in a width of two millimetres ; the second last col- 
umn includes all. 
