REPORT ON THE CRINOIDEA. 9 



the fourth and fifth, and the seventh and eighth brachials (PI. VII. fig. 2). The third 

 brachial is articulated by muscles and ligaments to those before and behind it, as arc 

 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 joints. They occupy a 

 curiously intermediate position between a bifascial articulation and a syzygy jaroper ; 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 Rliizocrinus, which come nearest to them in character, 

 being perfectly plain and simple, and not marked with radiating ridges as in the 

 Comatulse and some Pentacrinidse. But the trifascial articulation must not be 

 confounded with the peg and socket form of syzygy which is met with in Rliizocrinus 

 (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 Vllb. fig. 8, Id) 

 which is attached in a corresponding pit in the other face (PL Vila. figs. 16, 22, Id') ; 

 while in Rliizocrinus 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, 

 epais et bifurqud pres 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 parhinsoni, and it is diflicult to see what can have been lodged in the 

 fossa between the two limbs of the fork, except a third ligamentous bundle such as 

 occurs 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 III.; PL Vila. figs. 15, 17, 18, 19, 21,23; PL X. 

 figs. 1-4; PL XII. figs. 3-G, 8, 9, 12, 19, 20, 23; PL XXI. figs, la, ih, 2a, 2b, 

 Sb, 46, 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 Comatulse. But if the axillaries are 

 syzygial joints, there is always a muscular articulation below the hypozygal. No 

 1 Paleontologie Frangaise. Terrain Jurassique, t. xi. Crinoides, p. 309, pi. Ivi. fig. 2c. 



(ZOOL. CHALL. EXP. — PART XXXII. — 1884.) " 2 



