1896. | NEW YORK ACADEMY OF SCIENCES. 237 
of the Olenellus zone have but few rings (4 to 6), but in the 
Paradoxides zone, species with many rings (8 to 10 or even 12), 
are multiplied. The late Prof. C. F. Hartt recognized the dis- 
tinction between these two sections of the genus as it occurs in 
the Paradoxides beds, giving to one the name Dawsonia and to 
the other Eodiscus. The following scheme will show how the 
species of Microdiscus are related to each other: 
| | 
Lateral | Pygidial | Section LOBATUS Horizon. 
lobes of rachis has | 
pygid. 5 | 2£rings | lobatus, Hall, 1847* | rf 
ribbed D | 
e3e =} | | 
= yeni | 2 _ | unknown Meeki, Ford, 1876 
3 unknown | 'o= | | , | Olenellus 
| xibbed | 2 / Grings |  Parkeri, Wale. 1886 | Zone. 
| HO | 
= smooth | 3 ™) 6 rings Helena, Walc., 1889 
eee Section SPECIOSUS 
\eoene me 
| smooth | | 12 les | speciosus, Ford, 1874 | Olenellus 
Buon) | 10 rings | pellicinctus, Shaler &F, 1888; 22°: 
= —_— 2 —— 
=| | 
a é ee Section DAWSONIA f 
gs Fal 6to7 i ; | Paradoxides 
5 BY § S rings J Dawsoni, Hartt, 1868 lamellatus 
rae Ee Sub-zone. 
ae 8 6 rings sculptus, Hicks, 1871 
or ° 
n 
a) Section EODISCUS t{ 
3 ae Olenellus 
a 2 a.2| 6rings Schucherti, n. sp. Zone. 
2 2 "e 3. 
s _ 
eh mi d § a 6 rings connexus, Wale., 1887 olenel. : z.(?) 
e poe) Ba : Paradoxides 
a 2m 35 unknown precursor, Matt., 1885 bheteunia sare. 
a 5 12 rings suecicus, Linrs., 1882 | Sub-zone. 
oo aed 
a =! : | P. Abenacus 
5 cs z= é 12 rings pulchellus, Hartt, 1884 Aa ghee 
° ao oO = 
aQ idi ib 
3 9 rings eucentrus, Linrs., 1882 pee 
* First published as an Agnostus. 
+ Used by Hartt for M. Dawsoni, published by Dawson, 1868. 
{ Name used by Hartt (probably about 1870), not published until 1884. 
2 The type of the genus. 
