OF DENISON UNIVERSITY. 
83 
ing above the hinge line, anterior umbonal slope steep, a short but 
well-defined lunule, surface marked only by fine, crowded, concentric 
strice. Typical measurements as follows: 
Height at ^ from 
Thickness of 
Distance of umbo 
H’i 
ght to 
Length. 
Height. 
posterior margin. 
both valves. 
from anterior. 
length. 
(0 
25 
1 1 
8 
5-5 
7 
I 
2'27 
(2) 
24 
10 
8 
'y'l. 
5-5 
I 
2.40 
( 3 ) 
39 
16 
12 
10 
I 
2.43 
(4) 
18.5 
8 
6 
5 
I 
2-31 
( 5 ) 
26 
II -5 
8 
7 .3 
I 
2.26 
(6) 
24 
10 
8 ' 
6 
1 
2.40 
( 7 ) 
29 
13 
9 
8 
I 
2.23 
(8) 
27 
12 
9 
6 
I 
2.25 
( 9 ) 
29 
12.5 
1 1 
7 
I 
2.32 
(10) 
17 
8 
6 
.3 
I 
2.12 
(II) 
12 
6 
4-3 
3 
1 
2 
(13) 
12 
. l 5 
4 
3-5 
1 
I. I 18 
(14) 
10 
5 
3-5 
3 
I 
2 
Numbers 10-14 ^^e small specimens with slightly different char- 
acters. The lower margin is straight and the proportional length is 
less, while the convexity of the surface is more uniform. All the 
specimens are derived from the shale below the congl. I, and this spe- 
cies is so abundant and constant that it forms a valuable index to this 
horizon. It is known from many localities. The form is quite like 
Nuculana, but Prof. Winchell says: “This has the aspect of Cteno- 
donta hubbardi. Win., but is too thin and rather too much attenuated 
behind. However, I have thin specimens from Holland, near Lake 
Michigan.” 
Area ornata, sp. n. 
(Plate IX, Fig. 18. ) 
Shell small; rather convex, beaks near the anterior third, acute 
posteriorly, hinge short. Rather convex near the beaks, which are 
prominent and incurved; lower line nearly straight, as long as the 
greatest length of shell; anterior margin slightly curved or nearly 
straight, intersecting lower line by a sudden curve; posterior outline 
obliquely truncate forming at the lower angle a regular acute curve ; 
hinge short, inclined toward the posterior ventral angle ; anterior um- 
bonal ridge high, the slope making a strong angle with the surface of 
the valves ; post-umbonal ridge near the hinge. Surface marked by 
