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



canal ; while the fossas 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 Comatulaj 

 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 in the fossil Extracrvmis, Apiocrinus, &\idi Millericrinus. 

 It is likewise 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 ^iierced by the opening of the central canal, around which it is more promi- 

 nent than at its ends. These fossse 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 Vila. figs. 20, 22). It was wrongly described as a syzygy by 

 Sir Wyville Thomson in Bathycrinus aldricliianns,^ and also by Danielssen and Koren 

 in Bathycrinus carpenteri.* The vertical articular ridge is relatively large, and the two 

 fossae at its sides small in proportion (PL Vila. figs. 16, 20, 22, //'). But at its lower 

 end is a small though tolerably deep pit {Id'), which lodges a bundle of closely set liga- 

 ment-fibres corresponding to those forming the dorsal ligament in an ordinary muscular 

 joint (PL Vllb. fig. 5, Id). 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 lis;aments, viz., the two lateral ones 

 (//) and the median one on the dorsal side (Id) are all seen in section in PL Vllb. 

 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 Rhizocrinus 

 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 Oj}. a<.,pp. 715, 716. ' Journ. Linn. Soc. Land. (Zool.), vol. xiii. p. 50, 1876. 



* Ilyorinus carpenterii, Nyt Magazin for Naturvidenslcaherne, Bd. xxiii. pp. 6-8 (of separate copj"). 



