299 
[Foerste. 
part of the shell is moderate, and near the lateral margins, and 
still more towards the anterior margin, it becomes strongly convex, 
but scarcely geniculate. The length of one of the largest speci- 
mens is 14 mm. and the width 20 mm. 
At Collinsville, Alabama, this species occurs frequently, is of 
medium size, strongly corrugated and geniculate and shows fine 
muscular impressions in the ventral valve. Medium sized speci- 
mens are common at Hanover, Indiana, also at various localities 
in Ohio. Near Dayton, Ohio, a larger, more obese variety (to be 
described under the following notes on growth variations) occurs. 
The following notes are chiefly based on the Indiana collections. 
Small specimens 9 mm. long and 16 mm. broad, have a moderately 
convex ventral valve, curving rapidly anteriorly near the margin, 
forming a descending border 2 mm. long. There are either no con- 
centric wrinkles at all, or they are but faintly developed. Speci- 
mens 10 mm. long and 18 mm. broad, having a slightly increased 
convexity of shell, rounding anteriorly, strongly, but not abruptly 
into a descending border 3 mm. long. Concentric wrinkles distinct, 
but very low, appear over the entire valves, except on the genic- 
ulate border. Examining this area at a later stage, it will be read- 
ily seen that portions of the shell not wrinkled early in life become 
wrinkled, often quite strongly, as the shell reaches maturity. This 
wrinkling involves contraction of the shell antero-posteriorly, and 
this is readily permitted by its extreme thinness. When the shell 
has attained a length of 12 mm., and a width of 21 mm., a very 
strong wrinkle is often seen at the beginning of the anterior bor- 
der, which now may be called the geniculating ridge. The shell 
is still convex along the main body of the valve, but appears de- 
pressed within the borders of the geniculating ridge, on account of 
the moderate elevation of the latter. Anterior to the geniculating 
ridge, the border descends abruptly for a length of 3.5 mm. Ex- 
amining the later stages of the shell, it will be noticed that this 
geniculating ridge remains, but becomes more prominent, that the 
anterior border becomes gradually longer, that the shell posterior 
to the geniculating ridge remains convex but depressed just within 
the same, and that all this interior area is more or less strongly 
concentrically wrinkled. It becomes evident in measuring the speci- 
mens that the geniculating ridge continually assumes a more ad- 
vanced position than formerly, and that the old ridges apparently 
remain behind as so many concentric wrinkles. It is often possible 
