271 
[Foerste. 
cific distinctions. The pleurae are sharply defined, narrower than 
the intervening grooves, and show traces of tubercles arranged in 
rows longitudinally. One of these begins near the middle of the 
anterior pleurae, the other nearer the median lobe of the pygidium, 
and both rows converge posteriorly ; the inner pair of rows usually 
having reached the dorsal furrows at the fifth pleurae, beyond which 
this row does not usually extend. 
The following measurements show the variability of this pygid- 
ium. 
LENGTH IN MM. 
BREADTH IN MM. 
i 
K 
0 
5 
A 
P4 
o 
6 
A 
NO. OF SEGMENT 
AT WHICH FIRST TUBER- 
CLE IS SITUATED. 
1 
SECOND TUBERCLE. 
THIRD TUBERCLE. 
FOURTH TUBERCLE. 
FIFTH TUBERCLE. 
NO. OF SEGMENTS 
FIRST NO. DISTINCT. 
SECOND NO. INDISTINCT. 
10 
11.7 
8 
2 
6 
10 
14 
— 
15-21 
8 
9 
7 
- 
5 
9 
13 
- 
16-18 
9.2 
11 
8 
- 
5 
10 
14 
- 
16-19 
8 
9.2 
8 
3 
6 
10 
14 
18 
18-21 
8.5 
9.5 
8 
- 
6 
10 
14 
17 
17-21 
7.3 
8.5 
8 
2 
5 
9 
13 
- 
15-20 
7.3 
9 
7 
2 
6 
10 
- 
- 
13-19 
8 
9.5 
7 
— 
6 
10 
14 
— 
16-18 
Direct comparison with the pygidia from the siliceous bands of the 
Clinton at Lockport, New York, shows no distinctions, but it will 
be remembered that the eye-stalks of the Tennessee forms seemed 
longer. E. Thresheri , Foerste, has been separated from this species 
on insufficient grounds, since the pygidia alone are known. Only 
seven pleurae are found on the lateral lobes of the pygidium, but the 
eighth pleura of the Tennessee forms can often not be distinguished, 
and the number of pleurae on the pygidium of the New York spec- 
imens is often seven. Considering that only two or three speci- 
mens are known from the Clinton of Ohio, this distinction has 
scarcely local value, and for the same reason the numerical arrange- 
ment of the tubercles along the medium lobe of the pygidium means 
