REPORT ON THE CRINOIDEA. 9 
the fourth and fifth, and the seventh and eighth brachials (Pl. VII. fig. 2). The third 
brachial is articulated by muscles and ligaments to those before and behind it, as are 
also the sixth and ninth brachials. Beyond this point trifascial and muscular articula- 
tions alternate with one another throughout the arm. But those brachials which are 
united to their successors trifascially bear no pinnules as the remaining joints do; and 
in this respect they lose their morphological value as arm-joints, just as the hypozygal 
of a syzygium does. From this point of view, therefore, the description of the trifascial 
articulations as syzygia is perfectly correct. But they do not correspond to Miiller’s 
definition of a syzygy as an immovable sutural union of two joimts. They occupy a 
curiously intermediate position between a bifascial articulation and a syzygy proper ; for 
they resemble the former in the movement of the joints upon one another, and the latter 
in their occurring throughout the whole length of the arms, and in the absence of a 
pinnule on the lower joint of every pair so united. They correspond exactly in their 
distribution to the syzygies of Rhizocrinus, which come nearest to them in character, 
being perfectly plain and simple, and not marked with radiating ridges as in the 
Comatule and some Pentacrinide. But the trifascial articulation must not be 
confounded with the peg and socket form of syzygy which is met with in Rhizocrinus 
(Pl. X. figs. 1, 6, 8, 17, 18). In both cases there is a pit near the dorsal edge of 
one of the apposed faces; but in Bathycrinus this lodges a ligament (Pl. VIIb. fig. 8, dd) 
which is attached in a corresponding pit in the other face (Pl. VHa. figs. 16, 22, ld’) ; 
while in Riuzocrinus this other face bears a peg-like process (Pl. X. fig. 17) which fits 
into the pit, and thus checks rather than facilitates motion. 
It is noteworthy that there seems to have been a trifascial articulation between the 
two outer radials of the fossil Apiocrinus insignis, d’Orbigny, for the articular face of 
the second radial is described by de Loriol' as presenting “un bourrelet vertical large, 
épais et bifurqué prés du bord externe.” The fork of this ridge at its dorsal end gives 
the ‘joint-face an altogether different appearance from the corresponding part in 
Apiocrinus parkinsoni, and it is difficult to see what can have been lodged in the 
fossa between the two limbs of the fork, except a third ligamentous bundle such as 
oceurs in Bathycrinus. 
In all the Neocrinoidea muscular articulations occur between the first and second 
radials, between every axillary and the two joints which it bears, and between most 
of the following arm-joints (Pl. IIL; Pl. VIla. figs. 15, 17, 18, 19, 21,23; Pl X. 
fies 43) PL’ X11. figs._3-6,,8, 9; 12, 19, 20; 23; PL RE figa; la,;.1b;).20, 228: 
3b, 4b, 4c, 5c, 6d, &c.). When the arms divide and the axillaries are simple, they may 
be united by muscles to the preceding joints in Pentacrinus and Metacrinus (Pl. XII. 
fig. 3); though this is never the case in the Comatule. but if the axillaries are 
syzygial joints, there is always a muscular articulation below the hypozygal. No 
1 Paléontologie Francaise. Terrain Jurassique, t. xi. Crinoides, p. 309, pl. lvi. fig. 2c. 
(z00L, CHALL, EXP.—PART xxxi.—1884.) 2 
