8 THE VOYAGE OF H.M.S. CHALLENGER. 
canal; while the fossee for the attachment of the ligaments are on either side of it 
(Pl. VIla. figs. 7-11; Pl. X. figs. 11-14). 
A similar mode of articulation occurs between the two outer radials of most Comatulee 
and of a few species of Pentacrinus (Pl. XXX. figs. 11, 12; Pl. XXXII. figs. 16, 17; 
Pl. XXXIV. figs. 3, 6), as well as inthe fossil Hatracrinus, Apiocrinus, and Millerierinus. 
It is ikewise very common between the first two joints after the radial and every other 
successive axillary, in those species which have branching arms; and also between some 
of the lowest pinnule joints. It has often been incorrectly described as a syzygy or a 
modified syzygy, though clearly distinguished therefrom by Miiller* and by Dr. 
Carpenter.’ Each of the two apposed faces is divided into two lateral halves by a 
vertical ridge pierced by the opening of the central canal, around which it is more promi- 
nent than at its ends. These fossee lodge the strong interarticular ligaments, and no 
muscular bundles are interposed between the two joints. They are only capable of 
lateral movement upon one another, and cannot take part in any movements of flexion or 
extension, in which they act as a single segment only. 
A peculiar modification of this bifascial articulation, as it may be called, occurs in 
Bathycrinus. It is naturally best seen between the two outer radials (Pl. VIla. fig. 16) 
and the two lowest brachials, on account of their larger size; but it may be traced all 
through the arms (Pl. VIIa. figs. 20, 22). It was wrongly described as a syzygy by 
Sir Wyville Thomson in Bathycrinus aldrichianus,’ and also by Danielssen and Koren 
in Bathycrinus carpenteri.* The vertical articular ridge is relatively large, and the two 
fossee at its sides small in proportion (PI. VIIa. figs. 16, 20, 22, i’). But at its lower 
end is a small though tolerably deep pit (/d’), which lodges a bundle of closely set liga- 
ment-fibres corresponding to those forming the dorsal ligament in an ordinary muscular 
joint (Pl. VIIb. fig. 5, 7d). This bundle probably enables the two joints to take a 
larger share in the movements of flexion and extension than is possible in the bifascial 
articulations of the other Crinoids. The three ligaments, viz., the two lateral ones 
(i) and the median one on the dorsal side (/d) are all seen in section in Pl. VIb. 
fig. 8. 
A side view of a decalcified arm shows that there is a greater length of ligament 
between the two joints united in this way than there is between two joints which are 
united by syzygy in the much larger arm of a Pentacrinus; and though its length is but 
little greater than that of the fibres forming the syzygies in the arms of Ahizocrinus 
rawsoni, yet the latter are less numerous than in the trifascial articulation of Bathycrinus, 
especially on the upper (ventral) side of the central canal. 
In all the four species of Bathycrinus which are considered in this Report, a trifascial 
articulation, like that between the two outer radials, occurs between the first and second, 
1 Op. cit., pp. 26, 30. 2 Op. cit., pp. 715, 716. 3 Journ. Linn. Soc. Lond. (Zool.), vol. xiii. p. 50, 1876. 
4 Tlycrinus carpenterii, Nyt Magazin for Nuturvidenskaberne, Bd. xxiii. pp. 6-8 (of separate copy). 
