Foerste.] 
302 
[May 1, 
is 23 mm. long and 37.5 mm. broad and has a convexity of almost 
6 mm. The cardinal extremities slightly exceed the width of the 
shell. Only radiating striae are present. These are distinct, about 
four and one-half or five occupying a width of 2 mm. 
On a piece of Clinton limestone from Ohio (but from what pre- 
cise locality is unknown) belonging to the Ohio State University, 
a specimen is found precisely like the Indiana Clinton forms, ex- 
cepting perhaps for the fact that the curvature antero-posteriorly 
is more regular than in most Indiana specimens. It is a ventral 
valve 19 mm. long, 26 mm. broad along the cardinal line, and 5 mm. 
deep. The highest point on the valve is slightly posterior to the 
middle of the same. The very faint mesial depression near the beak 
is also noticed. The surface striae are the same. Another valve 
on the same piece of limestone, is very much flattened, the curva- 
ture being very moderate for a distance of 10 mm. from the beak 
then becoming a little more distinct for the remaining 6 mm. 
The mesial depression near the beak is also much more pronounced 
although still very shallow, being 5 mm. long and about 2 mm. 
broad. The surface striations are the same. 
Figure 31, PI. vm, Vol. n, Bull. Lab. Denison Univ., represents 
a specimen similar to the last from the Soldier’s Home quarry, in 
Ohio. It is also a flattened valve, the convexity being very mod- 
erate everywhere, but slightly stronger a little anterior to the mid- 
dle of the shell. In Indiana, specimens intermediate between the 
very convex and those in which the convexity is very moderate 
occur, so that it seems impossible to distinguish these shells on the 
basis of convexity alone. The character of the striae is the same. 
We believe they are both ventral valves. The dorsal valves we 
believe to be concave and one specimen seems very decidedly to 
corroborate this view. 
Two other suppositions are possible : the flattened valves might 
be dorsal valves, and the so-called dorsal valves might be the casts 
of ventral valves, and casts of the exterior are very difficult to dis- 
tinguish from the real surface of the opposite valves when the sub- 
stance of the shell itself is not preserved in any distinct and recog- 
nizable form. This, however, we do not believe to be the case, 
since intermediate forms are found and a quite satisfactory con- 
cave valve is known. 
The flattened valve might also be a young dorsal valve of Strep - 
torhynchus tenuis , Hall, but the striae of that species are coarser 
