External Structure of Paleozoic Crinotds. 385 
ventral furrow, which continues from the arm-bases up to the 
tips of the arms and along the pinnule. The pinnule spring 
out alternately right and left from the arm-plates, their fur- 
rows connecting with that of the arm and forming, in fact, a 
continuation of the same. 
The furrow appears, in specimens in ordinary preservation, 
as a simple groove, which communicates through the arm- 
openings with the inner cavity of the body. Only in rare 
instances has the furrow been found covered by minute plates, 
whose construction, however, has heretofore not been ascer- 
tained. ‘The best specimen of this kind that I have seen is 
a Cyathocrinus malvaceus, in which the little plates above the 
furrow can be studied in all their details with the greatest 
precision. ‘The specimen is the property of Frank Springer, 
Esq., who had the kindness to leave it with me for investi- 
gation and description. ‘The arms of Cyathocrinus are com- 
posed of long slender joints with a wide ventral furrow. They 
bifurcate frequently, each branch bifurcating at intervals 
again. ‘There appears on the arms of Cyathocrinus no scar for 
the attachment of pimnule; and as these appendages have never 
been observed in the genus, it is probable that the many little 
branches performed their functions. In Mr. Springer’s speci- 
men the plates above the furrow consist of two rows of minute 
pieces on either side, the inner rows of which join in the 
middle, interlock with each other, and form an apparently 
solid covering. The outer plates which are attached to the 
arm-joints are toward the upper end of the arms placed partly 
upon the edges of the joint, but nearer the calyx rest wholly 
against the edges of the upper part of the ventral furrow. 
They are longitudinally arranged, partly hidden from view 
by the inner plates. he visible part is quadrangular, with 
a narrow tooth-like projection toward the lower end of each 
plate, which is directed inward and slightly downward as a 
sharp, elongated process, and forms a support for the mner 
plates. The inner plates are elongated triangular, resting 
with their shorter sides against the inner faces of the outer 
series, and, between the tooth-like extensions, overlapping 
them with their bevelled lateral edges, in such a manner that 
each plate exteriorly fits in and fills the space between each 
pair of similar triangular plates on the other side. The two 
longer sides interlock with corresponding sides of similar plates 
of the opposite row, their sharp angles or apices meeting the 
sutures between the opposite quadrangular plates. At each of 
these points of junction, just beyond the apex of each triangular 
piece, on either side of the furrow, there is a little pore, which 
evidently communicates with the inner channel. There are 
Ann. & Mag. N. Hist. Ser. 5. Vol. 1. 26 
