No. 1618. THE CRINOID GENUS EUDIOCRINUS—CLARK. 273 
The Ludiocrinus indivisus group is peculiar in lacking the pinnule 
on the third joint “ above the radial, while a pinnule is present on the 
joints on either side of it; the explanation, however, is simple; the 
first two joints beyond the radials represent the two costals of a ten- 
armed or multibrachiate comatulid; they are somewhat broader than 
the following, and are united by syzygy as in Zygometra, the third 
joint, therefore, represents the first free brachial of a ten-arined form, 
and, as the first free brachial never bears a pinnule, we find it absent 
here also; the fourth joint (representing the second free brachial) 
bears a pinnule, and the fifth and sixth (i. e., third and fourth free 
brachials) are united by syzygy, as usual in all comatulids. Thus 
the species of the Hudiocrinus indivisus group are apparently derived 
by a reversion from a ten-armed type; and in this connection it is 
interesting to call to mind the specimen upon which Carpenter’s 
“Antedon clemens” was based, which had one undivided ray, with a 
pinnule on the “second brachial,’ but none on the “third,” exactly 
as in the Z. indivisus group. No one can doubt that the single un- 
divided arm in A. clemené is a derivation from a normally divided 
arm, and, as the arms in the /. indivisus type are similar in structure, 
it is reasonable to suppose that they, too, are derived from more com- 
plex arms. | 
Moreover, taking the type of articulation into consideration, the 
second and third (postradial) joints are joined in the same way as 
the first is joined to the radial; that is, by articulating faces made 
up of (dorsally) a large fossa lodging the dorsal ligament, ven- 
tral to which, on each side of the axial canal, hes a pair of fossa 
iodging the interarticular hgaments, and beyond them the muscular 
fossee; the third and fourth (postradial) joints are joined by an ar- 
-ticulation consisting only of a pair of large pits lodging the inter- 
articular hgament, separated by a ridge at right angles to the long 
(transverse) axis of the joint face, this constituting what has been 
called a synarthrial joint; the fourth and fifth joints are joined by a 
type of articulation first found between the second and third free 
brachials of most comatulids, namely, a modified form of that be- 
tween the radials and next following joints, in which the transverse 
ridge separating the dorsal-hgament fossa from the fosse of the 
interarticular ligaments is strongly diagonal, so that one end is dorso- 
lateral in position and the opposite end ventro-lateral; the interartic- 
“Tn Bells figure of the arm base of Hudiocrinus granulatus (Proc. Zool. Soe. 
London, 1894, Plate XXIII, fig. 3) the first pinnule is omitted ; he describes the 
species aS having “the first pinnule on the left side of the second brachial ;” 
but according to his figure it must have been on the right side, as the second 
pinnule (the lowest shown) is on the left. Attention should be called to the 
fact that there is a serious discrepancy between the size of H. granulatus as 
described and as figured. 
Proc. N. M. vol. xxxiv—08——18 
