IOWA ACADEMY OF SCIENCES. 
179 
During a recent study of the fossils of Missouri most of the 
species hitherto described were obtained and much new material 
was examined, some of it being in an unusually fine state of 
preservation. In a number of cases the structure of the organ- 
isms was made out much better than ever before, enabling crit- 
ical comparisons to be instituted between most of the known 
genera. The minute structure of the ambulacral plates and 
ornamentation of Melonites, Oligoporus, and Archmocidaris 
were especially well exhibited. The more salient points of dif- 
ference are shown in the accompanying plates. 
It is the purpose of the present paper to bring together the 
material which has been described and to present briefly some 
of the more salient characteristics of the group. It is prepar- 
atory to a more complete and critical study of all the material 
now known, and a careful comparison of the genera with typ- 
ical European forms as well as with those most nearly related 
among the Mesozoic and later groups. So much American 
material has accumulated of recent years and is now within 
easy reach that much of interest is promised as the result of its 
examination. 
The American Paleozoic echinoids with a single exception are 
from the Carboniferous. Although during the period there was 
a remarkable and important expansion of echinodermal life, and 
a wonderful culmination of certain groups, echinoids were 
scarce. The genera were few, and species were meagerly rep- 
resented and widely scattered, though they showed a constant 
yet slow increase from the time of their first appearance. The 
distribution of the different genera and species is indicated in 
the following table: 
GEOLOGICAL DISTKIBUTION OF AMERICAN PALEOZOIC ECHINOIDS. 
It is to be noted that the echinoids occur most abundantly in 
the Augusta limestone, the rocks in which another group of 
