REPORT ON THE CRINOIDEA. 233 



trifascial articulations; while the third, sixth, and ninth brachials are usually free, i.e., 

 united by muscles to the joints before and behind them, as the joints of a Crinoid arm 

 gendi-ally 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. VIII). 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 (PI. 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). An arm 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 cddrichianus and Bathycrinus campbelUanus 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 every 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 Vila, 

 fig. 21; and a corresponding joint-face of Bathycrinus campbellicmus in fig. 23. In the 

 latter type the pinnide-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 camiiibellianus 

 (PL VIII. fig. 5) and to a less degree in Bathycrinus aldrichianus (PL VII. figs. 2, 7). 

 It is also figured by Danielsson and Koren^ in Bathycrinus carpenteri. The first six or 



1 Nyt May. f. Naturvidemk., Bd. xxiiL, Tab. L fig. 14. 



(ZOOL. CHALL. EXP. — PART XXXII. — 1884.) li 30 



