10 THE VOYAGE OF H.M.S. CHALLENGER. 



muscles are ever found between the second and axillary radials of any Neocrinoid except 

 Eudesicrinus (p. 215), nor (except in Metacrinus) between the first and second brachials, 

 unless the latter be a syzygial joint. In the aberrant Metacrinus, however, the second 

 radial is a syzygial joint (PL XII. figs. 7-10) and the axillary is usually either the fourth or 

 the sixth radial (PI. XII. figs. 3, 4; PL XXXIX. fig. 1 ; Pis. XLIL, XLIIL, XLV., XLVL, 

 XLVIII.-LL); and there is usually a syzygy in the third joint after each successive axillary, 

 just as in the free arms of most ComatulEe. But the first two joints are united by muscles 

 instead of by ligaments, as in the Comatulse ; and I have not met with any instances of 

 bifascial articulation in the arms of this genus, which have nothing but syzygies and 

 muscular unions like the arms of Actinometra Solaris or Actinometra typica. 



Each pinnule has a muscular attachment to the arm-joint which bears it (PI. Vc. 

 fig. 2, m; PI. Vila. fig. 21; PL XVII. fig. 1; PL XXXa. figs. lOa, 12«; PL XLI. 

 fig. 11 ; PL XLVII. figs. 11, 12) ; while in Metacrinus and some tropical Comatulse the 

 lower joints of the pinnules are united by muscles instead of by ligaments or suture 

 only (PL XIII. fig. 12). 



The articular face of a joint which is connected with its successor by muscular 

 bundles presents a variety of fossai separated by intervening ridges, that are frequently 

 somewhat ill defined (PL Vila. figs. 15, 17, 18, 19, 21, 23; PL X. figs. 1-4; PL XII. 

 figs. 3-6, 8, 9, 12, 15, 19, 20, 23; PL XXL figs. la. ih, 2a, 2b, Sb, Ab, 5c, Qd, &c.). 

 Beneath its upper and lateral edges lie the two fossae in which the muscular bundles are 

 attached (PL Vila. fig. \^,,rm'). They are sometimes separated by a vertical ridge as 

 on the first radials of Antedon lyhalangium, Antedon eschrichti, and of most species of 

 Antedon and Prmnacliocrinus ; and sometimes by a groove which may extend down- 

 wards to the articular rim around the opening of the central canal, as in the first radials 

 of Antedon rosacea and most species of Actinometra and Pentacrinus (PL XXI. fig. Qd). 

 In these last types the fossae are comparatively small ; but in Promachocrinus and in 

 most species of Antedon they occupy nearly or quite half of the articular face, as in 

 Bathycrinus aldrichianus (PL Vila. fig. 15, rm' ; PL Vllb. fig. 5, rm). Below the 

 muscular fossae, and separated from them by more or less distinct ridges, are those 

 lodging the interarticular ligaments (PL Vila. fig. 15, W ; PL Vllb. fig. 5, and 

 PL Villa, fig. 7, li). They are bounded below by the transverse articular ridge, which is 

 pierced by the opening of the central canal. This is usually surrounded by a more or 

 less prominent rim, from which the ridges start that separate the muscular fossae from 

 the ligamentous ones and from one another. All these characters arc much less distinct 

 in the Pentacrinidse than in the ComatulfB. 



Beneath the transverse ridge, and bounded below by the curved edge of the articular 

 face is a large fossa extending right across the face and lodging the dorsal elastic 

 ligament (PL Vila. fig. 15, Id'; PL Vllb. fig. 5, and PL Villa, fig. 7, Id). As a 

 general rule this ligament is chiefly concentrated in a small, more or less oval j^it situated 



