Foerste.J 
304 
LMay 1, 
the shell or slightly produced, forming small acute ears ; the sides 
subparallel posteriorly, anteriorly rounding into the semicircular 
anterior margin of the shell ; occasionally with faint, almost ob- 
solete folds along the cardinal margin, corresponding in direction 
to the lateral margins of the acute ears when present. 
Dorsal valve flat in some specimens forming almost a plane sur- 
face. In one specimen, a cast, there are depressions correspond- 
ing to two short cardinal teeth forming an angle with one another 
of about one hundred and twenty degrees, and a third depression 
between about twice as long as the teeth, corresponding to a mesial 
ridge. 
Ventral valve flattened, moderately convex, greatest convexity 
near the beak, thence sloping gradually towards the front and sides 
and more rapidly towards the postero-lateral margin. In casts of 
this valve two depressions are found forming an angle of about 
eighty degrees with one another, becoming indistinct a short dis- 
tance from the margin. These correspond to the cardinal teeth of 
the ventral valve, outlining the posterior sides of the muscular 
depressions. 
VALVE. 
D, dorsal; v, ventral. 
LENGTH. 
WIDTH. 
RATIO OF 
LENGTH TO WIDTH. 
NUMBER OF 
STRIJE IN TWO MM. 
H 
25 
W 
25 
5 a 
> 2 ^ 
f-S® 
MO 
a 
o 
V. 
14.8 
17.6 
.841 
9 
3—4 
V. 
13.0 
15.5 
.838 
9—10 
4 
d. 
18.7 
22.6 
.827 
8 — 9 
4-6 
V. 
17.1 
21.0 
.814 
8—9 
4-6 
V . 
20.0 
24.9 
.803 
8-9 
3-4 
d. 
16.2 
20.2 
.802 
9—10 
5-8 
V. 
14.7 
18.5 
.794 
10-11 
4—5 
d. 
19.1 
24.3 
.786 
9 
4—6 
d. 
22.7 
29.0 
.783 
10-11 
4—5 
d. 
16.0 
20.5 
.780 
9—10 
5-8 
d. 
- 
18.8 
— 
10—12 
6—11 
The surface of both valves is covered with fine radiating striae, 
