178 
IOWA ACADEMY OF SCIENCES. 
SYNOPSIS OF AMEEICAN PALEOZOIC ECHINOIDS. 
BY CHARLES ROLLIN KEYES. 
For the more minute correlations in stratigraphy no group 
of fossils is of more practical service than the echinoderms. 
Where the remains of these organisms are abundant, as in many 
of the Paleozoic rocks, the entire history of events may be 
inferred from them alone. Composed of hard parts or plates 
which are more or less regularly shaped and which are arranged 
according to a definite plan the evolution of the various forms 
may be readily traced and the slightest changes in physical 
-conditions at the time of sedimentation quickly detected. In 
the Mississippi basin the crinoids are widely distributed both 
geographically and geologically in the later Paleozoic strata. 
Their structure has been made out in great detail and hence 
their value for geological classificatory purposes is second to no 
other criteria. The same is also true of a closely related class 
of stemmed echinoderms — the blastoids. 
There is another group of echinoderms which begins to 
appear rather frequently in the latter part of the Paleozoic and 
which continues to become more and more abundant through 
the Mesozoic to the present time. These are the echinoids, pop- 
ularly known along the present seacoast as sea eggs or sea 
urchins. The recent species have been lately exhaustively 
monographed by Prof. Alexander Agassiz. The Mesozoic 
forms have, within the past year, received scholarly treatment 
at the hands of Prof. W. B. Clark; and the Tertiary varieties 
are now being thoroughly studied by the same author. 
With the American Paleozoic echinoids practically nothing 
has as yet been done to place this group on a working basis. 
To be sure, a considerable number of species have been named, 
but with very few exceptions the descriptions have been very 
meager and have been founded upon exceedingly fragmentary 
material — isolated plates or primary spines. 
