REPORT ON THE. CRINOIDEA: 233 
trifascial articulations ; while the third, sixth, and ninth brachials are usually free, ée., 
united by muscles to the joints before and behind them, as the joints of a Crinoid arm 
generally are. In Rhizocrinus, however, the two joints of each successive pair are united by 
syzygy, so that muscular articulations and syzygies alternate regularly all along the arm. 
But as regards the last point Bathycrinus approaches Rhizocrinus much more closely than 
was supposed by Sir Wyville Thomson. Instead of there being one syzygy (trifascial 
articulation) only, or two at the base and others scattered sparingly at irregular intervals, 
there is just as much regular alternation after the ninth brachial as there is throughout 
the whole arm of Rhizocrinus. In fact, in some irregular arms of Bathycrinus the third 
brachial is the only joint which has muscles attached at both ends, the alternation which 
would ordinarily commence with the tenth appearing on the fourth and following joints. 
Except in Bathycrinus aldrichianus the bases of the arms are but little wider than 
the succeeding portions. The flattening of the dorsal surface at the sides of the median 
ridge, which commences on the two outer radials, is continued on to the first two or 
three brachials and then disappears (PI. VIIb. figs. 7, 8). The same is the case with the 
sharp lateral edge which is so marked on the second and axillary radials. In Bathycrinus 
aldrichianus it is continued out on to the first eight arm-joints (Pl. VII. fig. 2), and 
marks the line of junction of the curved dorsal surface and the side faces, which slope 
upwards and inwards towards the medio-ventral line (Pl. VII. fig. 8). Anarm of this 
species, 30 mm. long, consists of fifty joints ; but the first seven or more bear no pinnules. 
In most cases the first pinnule is borne on the eleventh joint, when this, as is normally 
the case, has a muscular articulation at its distal end. The ninth brachial is usually a 
joint of this kind, and in Bathycrinus aldrichianus and Bathycrinus campbellianus some- 
times bears the first pinnule, while the eighth may do so. But the joint which bears a 
pinnule is invariably united to its successor by muscles, so that the pinnules only occur 
upon eyery alternate joint through the whole length of the arm, exactly as in Rhizocrinus. 
The only difference is that the joints which do not bear pinnules are united to those 
which do by syzygies in Rhizocrinus, and by trifascial articulation in Bathycrinus. The 
distal face of a pinnule-bearing joint of Bathycrinus aldrichianus is shown in Pl. VIIa. 
fig. 21; and a corresponding joint-face of Bathycrinus campbellianus in fig. 23. In the 
latter type the pinnule-socket is more at the side of the joint and less at the end than in 
Bathycrinus aldrichianus, 
The pinnules of Bathycrinus (Pl. VII. fig. 7; Pl. VIII. fig. 5) are moderately short 
and slender, and composed of relatively few joints. The. lateral edges of these joints, 
especially the outer ones, are produced upwards at the sides of the medio-ventral furrow 
so as to protect it very completely. This is most marked in Bathycrinus campbellianus 
(Pl. VIIL. fig. 5) and to a less degree in Bathycrinus aldrichianus (Pl. VII. figs. 2, 7). 
It is also figured by Danielssen and Koren’ in Bathycrinus carpentert. The first six or 
1 Nyt Mag. f. Naturvidensk., Bd. xxiii., Tab. i. fig. 14, 
(ZOOL, CHALL, EXP,—PART XXx11.—1884.) Li 30 
