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



PI. XXXII. figs. 1, 2 ; PI. XXXVII. figs. 5-8, 19, 21 ; PL XXXIX. figs. 3, 5, 6 ; PI. XLI. 

 figs. 5, 15 ; PI. XLV. figs. 3, 5, 6 ; PI. XLVII. figs. 1-3 ; PI. L. figs. 4, 20, 21 ; PI. LI. 

 figs. 5-8). 



The external line of separation frequently disappears altogether, or is only traceable 

 with great difiiculty, and the two joints, primitively separate, become practically fused 

 into one (PI. XLII. fig. 1 ; PI. L. fig. 3). I cannot find any mention of this peculiarity 

 in the classical memoir of Johannes MllUer,' who spoke of the successive unions of 

 the stem-joints indifferently as " Nathe oder Gelenke." So far as I can make out, it 

 was first noticed by Sir Wyville Thomson in Pentacrinus decorus ;^ and Liitken' 

 subsequently described it in more detail in Pentacrinus asteria and Pentacrinus mUlleri. 

 Quenstedf* also noticed it in the fossil species Pentacrinus scalaris and Pentacrinus 

 jurensis. 



An essentially similar mode of union between certain of the arm-joints was spoken 

 of by Miiller as a " syzygy," and described as an immovable sutural union. The name 

 syzygy has since been applied to the sutural union of the nodal stem-joints with those 

 next below them ; and Miiller's terms " hypozygal " and " epizygal " for the two arm- 

 joints which are united by syzygy (PL XII. figs. 7, 10, 18, 21 ; PL XXXa. figs. 9, 10, 12 ; 

 PL XXXII. figs. 4, 9, 15, 18) may be conveniently applied in the case of the stem-joints 

 also. 



In all the recent ComatulsB the apposed faces of the two portions of a syzygial joint 

 are marked by a series of slightly elevated ridges with alternating furrows, which radiate 

 from the opening of the central canal towards the dorsal margin of the joint. In 

 Actinometra typica these ridges are frequently not perfectly continuous ; but they are 

 broken up into a row of little elevations, squarish or oblong in shape, and arranged with 

 their longer axes radiating outwards from the central canal. On some joints these are 

 not very numerous, and as their terminal faces are marked by median vertical lines, they 

 have been wrongly described as surfaces effecting a ligamentous articulation of the 

 bifascial type, such as will be described immediately.' 



The radiating arrangement is usually much less marked in the Pentacrinidse than in 

 the Comatulse, the striation being frequently only visible at the extreme marginal 

 portions of the syzygial surfaces (PL XII. figs. 7, 10, 18, 21 ; PL XXI. figs. Id, 2d, 

 5a; PL XXX. figs. 20, 21) as figured by Miiller in Pentacrinus asteria,^ while in 

 some cases it appears to be absent altogether, the apposed faces being perfectly smooth 



1 Ueber den Bau des Pentacrinus caput-Mediiste, Ahlmndl. d. k. Akad. d. IViss. Berlin, 1843. 



2 Sea Lilies, The Intellectual Observer, No. 31, August 1864, p. 7. 



' Om Vestindiens Pentacriner, med nogle Bemaerkninger om Pentacriner og Sdlilier i Almindelighed, Vidensk. 

 Meddel.f. d. nat. Foren. i Kj^henhavn, 1864, Nos. 13-16, pp. 198, 199. 



^ Petrefactenkunde Deutschlands, Bd. iv., Asteriden und Encriuiden, pp. 196, 230, Taf. 98, figs. 2, 3, 107. 



^ See Lov^n, Phanogenia, ett liittills okiindt slagte af Ma Crinoideer, Ofversigt k. Vetensk.-Akad. Forhaiidl., 

 Arg. xxiii.. No. 9, p. 230, fig. c; and also P. H. Carpenter, The Comatulre of the Leyden Museum, Notes from the Leyden 

 Mtiseum, vol. iii. pp. 197-199. ^ Op. cit., Taf. ii. fig. 4. 



