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



wliicli the upward direction of the cirri and the groo-sdng of the supra-nodal joint ae 

 tolerably constant characters (PI. XXXVIII. ; PI. XXXIX. figs. 3, 9 ; Pis. XL., XLIL, 

 XLV. ; PI. XLVII. figs. 1, 2 ; PL XLVIIL). 



Another small point of resemblance between the European Pentacrinus and the 

 Pacific Metacrinus is the slight tendency sometimes showTi by the basals of the former to 

 send median downward extensions over the interradial ridges at the top of the stem 

 (PI. XVIII. fig. 2), for this character is very generally distinctive of Metacrinus 

 (PL XXXIX. fig. 1 ; PL XLIII. figs. 1, 3 ; PL XLVIIL). The basals of Pentacrinus 

 wyville-thomsoni are almost always markedly pentagonal, the height being decidedly 

 greater in the midtUe than at the sides, where, however, it is usually distinctly appre- 

 cialile (PL XVIII. fig. 3 ; PL XIX. figs. 1, G, 7 ; PL XX. fig. 3). Sometimes, however, 

 they become almost triangular in outline (PL XVIII. figs. 1, 2), and one or more of them 

 occasionally fail to meet their feUows, a variation which is more frequently met wdth in 

 Pentacrinus mUlleri (PL XV. figs. 1, 2) and Pentacrinus naresianus (Pis. XXVIIL, 

 XXIX.). 



The number of arms in Pentacrinus wyville-thomsoni is comparatively smaU, 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, rarel}^ 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 ofi" from the lateral calcareous band, especially 

 at the bases of the pinnules ; and this band itself is but very imperfectly difi"erentiated 

 into side plates (PL XVII. figs. 2, 3). 



Unlike many of the Pacific Pentacrinidse, 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 kiucUy 



