320 THE VOYAGE OF H.M.S. CHALLENGER. 
which the upward direction of the cirri and the grooving of the supra-nodal joint ae 
tolerably constant characters (Pl. XXXVIII.; Pl. XXXIX. figs: 18;.0'5 (Pls ka ea. 
ALY. ; PL XLVII. figs: 1, 2) BE Ravine): 
Another small point of resemblance between the European Pentacrinus and the 
Pacific Metacrinus is the slight tendency sometimes shown by the basals of the former to 
send median downward extensions over the interradial ridges at the top of the stem 
(Pl. XVII. -fig. 2), for this character is very generally distinctive of Metacrinus 
(Pl. XXXUX: figs T 5 PL OLa i ehieseels Be) Pl, XLVIII.).. The basals of Pentacrinus 
wyville-thomsoni are almost always markedly pentagonal, the height being decidedly 
greater in the middle than at the sides, where, however, it is usually distinctly appre- 
elable (EL XVM fie 3.5 PLAS thes a6, 7; PL XX. fig. 3). Sometimes, however, 
they become almost triangular in outline (Pl. XVIIL. figs. 1, 2), and one or more of them 
occasionally fail to meet their fellows, a variation which is more frequently met with in 
Pentacrinus miilleri (Pl. XV. figs. 1, 2) and Pentacrinus naresianus (Pls. XXVIIL, 
XXIX.). 
The number of arms in Pentacrinus wyville-thomsoni is comparatively small, being 
sometimes as low as fourteen ; for two or even three of the rays may have no axillary 
but the third radial, as is sometimes the case in Pentacrinus decorus (Pl. XXXV.); and the 
distichal axillaries, when present, rarely occur all round the cup (Pl. XVIII. fig. 3). The 
examples figured in Pl. XVIII. figs. 1, 2, and Pl. XIX. figs. 1, 6, 7, are some of those 
with the greatest number of arms, a tertiary (palmar) axillary being occasionally present 
beyond the distichal ; but I do not know of any specimen in which the number of arms 
exceeds twenty-two. : 
The disk (Pl. XVII. fig. 6) is closely covered by a pavement of anambulacral plates, 
several of which are pierced by water-pores. These are almost entirely absent in the anal 
interradius, in the proximal part of which the plates are closer set than usual, and arranged 
into two lateral groups. At first sight these look like large single plates, and are 
suggestive of orals, but they become resolved on further examination into small and very 
closely set plates. The ambulacra of the disk are protected by irregular plates which 
cover them in completely in the dry state. They are more regularly arranged on the 
arms, but are discontinuous on alternate sides between the pinnule-bases (Pl. XVII. fig. 4) ; 
and the perisome covering the muscular bundles is likewise plated, as in the allied 
Pentacrinus alternicirrus (Pl. XXVII. fig. 6). The covering plates of the pinnule- 
ambulacra are not very distinctly marked off from the lateral calcareous band, especially 
at the bases of the pmnules; and this band itself is but very imperfectly differentiated 
into side plates (Pl. XVII. figs. 2, 3). | 
Unhke many of the Pacific Pentacrinide, which were white when fresh, living 
examples of Pentacrinus wyville-thomsoni have a beautiful grass-green colour. This 
becomes duller in spirit, which acquires a purplish-red tinge. Prof. Moseley kindly 
