258 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
there has been called I. maximus. As I. cowensis has certain char- 
acters half-way between I. gigas and I. maximus, this misidentification 
has served to throw the Cincinnati specimens into hopeless confusion, — 
The following key will show the more important characters which 
separate the genera of asaphids occurring in the formations from the 
Chazy to the Richmond. 
A. Depressed or concave border on both shields. 
a. Surface of shields ribbed. 
1... Free cheeks meeting in front... 2.+.4.4¢ 0 nee Ogygites, 
2. Free cheeks separated in front................ Basilicus. 
b. Surface of shields not ribbed. 
1... Axial lobe narrow;.prominent. |. 2.25/40 Tsoteloides. 
2. Axial lobe wide, depressed: ... 2.2... 52.22 Tsotelus. 
B. Concave border on pygidium only. 
a. | Byes very large.is..).di. cee) Oe. Nileus. 
b.,, Eyes small but :very hight . sc)... sein eee V ogdesia. 
C. No concave border on either shield. 
a. Free cheeks meeting in front................ Onchometopus. 
b. Free cheeks separated in front................ Brachyaspis. 
The four more common species of Isotelus in the Middle and 
Upper Ordovician can be separated as follows :— 
Se ee 
A. Shields about three fourths as long as wide. 
a. Adult without genal spines. Shields subtriangular. .J. gigas. — 
b. Adult with genal spines. Shields rounded..:...J. iowensis. 
B. Shields less than three fourths as long as wide, regularly rounded. 
a. Adult ‘without ‘genal’ spies... 0... .....2.2 5 on ee I. latus. 
b.., Adolt with,penal. spies.) Jone vue su I. maximus. 
This may at first sight seem to be an arbitrary arrangement of the 
species, but these are not the only characteristics in which the above 
species differ, the other points being given in the general discussion 
of each species. The geographical distribution is also in agreement | 
with the above separation. For instance, the specimens which I have 
described as Isotelus latus were considered by Billings the female of 
I. gigas, but if this is true, why are all the hundreds of specimens found © 
at Trenton Falls, the narrow form? 
a Ee 
. 
I have measured an extensive series of each species, and find the | 
ratio of length to the width of the shields to be an important clue to _ 
their identification. From the above key, it might seem that it would 
