On the Structure of Paleozote Orinotds. 453 
LI.—Notes on the Internal and External Structure of 
Paleozoic Crinoids. By CHARLES WACHSMUTH*, 
[Continued from p. 392. } 
5. The Construction of the Summit, and its Value in 
Classtfication. 
The construction of the ventral disk or actinal side of the 
calyx has heretofore received less attention than almost any 
other part of the Crinoids; and thereby an important aid to 
classification has been overlooked. I think it affords a clear 
and most important distinction between recent and ancient 
Crinoids, and shows that they fall naturally into two great 
divisions or groups. ‘This view, although it does not agree 
with the opinion of other authors, who, in their classifications, 
have placed a number of Paleozoic genera in the same 
group with the recent Crinoids, is, as I hope to show, well 
founded. 
Dr. F. Roemer, in the ‘ Lethea Geognostica,’ 1855, p. 227, 
divides “‘ the true Crinoids, which are supported by an arti- 
culated or jointed column,” into two divisions :— 
a. Crinoids in which the ventral side consists of a soft skin. 
b. Those in which the ventral side is covered by solid im- 
movable plates. 
Roemer includes with the former group Pentacrinide, Apio- 
erinide, Eugeniacrinide, Encrinide, Cigbshnberintdes and 
Cyathocrinide. This division seems to have been based on 
mere conjecture, since a membranous ventral surface has been 
observed only in the Pentacrinide and the recent Crinoids 
generally, though it is probable that Hugenitacrinus and seve- 
ral allied genera had that summit structure. In the Apio- 
crinide and Encrinidz, however, the general construction of 
the dorsal or abactinal parts, the massive plates, both of 
calyx and arms, indicate rather a closer relationship with the 
ancient Crinoids, and suggest the existence of a solid dome. 
The latter becomes more probable since a solid vault has 
been discovered in Belemnocrinus. This genus is in its 
generic formula and general form almost identical with the 
recent Rhizocrinus, which, on the contrary, is covered by a 
soft peristome. Both are closely related to Apiocrinus; Be- 
lemnocrinus particularly has the same heavy body-plates and 
the small visceral cavity ; and it appears to me that Apdocri- 
nus is more nearly allied to the Paleozoic type than to the 
recent Rhizocrinus. 
The Cupressocrinide and Cyathocrinide are the only groups 
from Paleozoic formations which Roemer places in his divi- 
* From ‘Silliman’s American Journal,’ Sept. 1877. 
