1889 .] 
301 
[Foerste. 
as a rule, these species would be difficult to distinguish, were it not 
for the fact that neither the convexity of the ventral valve, nor the 
point of strongest concavity of the dorsal valve lies ever so near 
to the cardinal line in S.patenta as in S. striata. In other words 
S. patenta remained for a longer time a shell of simple curvature. 
Several flattened dorsal valves of the S. patenta type which have 
a sickly appearance have been found at Hanover, Indiana. The 
shell is concave near the cardinal margin, the curvature changing 
to convexity at about 9 mm. from the cardinal line, convexity and 
concavity together producing a flattened shell. Anteriorly along 
the median line, there is a bend in the shell producing there a low, 
undefined median elevation. The surface is marked by radiating 
striae. It is possible that these may be sickly distorted forms of 
S .patenta (Plate iv, fig. 2). 
Strophomena Hanoverensis, sp. nov. 
(PLATE VI, FIG. 1.) 
Among the Clinton Group fossils found at Hanover, Indiana, 
is a species perhaps most closely related to S. profunda, Hall. The 
dorsal valve is concave, and the ventral valve convex from the 
very first, and remains so through life, increasing however in de- 
gree of curvature with age. It will be seen then that in their young 
stages S. rhomboidalis , S. patenta and S. Hanoverensis, all agree 
in having concave dorsal and convex ventral valves of simple char- 
acter, and that other variations result from subsequent processes 
of growth. S. profunda has a low but readily recognized beak on 
the ventral valve. The corresponding surface of the ventral valve 
in S. Hanoverensis is flattened or even contains a very slight me- 
dian depression of very short length. This feature will readily 
distinguish the Clinton species of Indiana from the species imme- 
diately above in the Niagara. 
The most elevated or convex portion of the ventral valve varies 
in position from slightly more than one-half to slightly less than 
four-fifths the length of the shell from the beak. This simply 
means that the greatest convexity if not elevation of the shell is 
about 9 mm. from the cardinal margin and that the anterior parts 
of the shell thereafter are less strongly convex. Owing to this 
reason also the general height of the shell is not proportionate 
to its size. A medium sized specimen 19 mm. long and 25 mm. 
broad has a convexity of almost 5 mm. The largest specimen seen 
