20-1: THE VOYAGE OF H.M.S. CHALLENGER. 



(PI. I.) is almost bare, though faint elevations are visible along the middle lines of the 

 radials, and indistinct, scattered tubercles appear between them, except in the centre 

 of the trivium. In the smaller specimen, however (PI. II.), there are three, fairly distinct 

 double rows of blunt tubercles which correspond to the trivial radials. But on the 

 l)ivial side there is more indication of median ridges; while in the fragment shown in 

 woodcut (fig. 10) the double row of tubercles is tolerably distinct all round the cup, except 

 on one of the bivial radials. Other tubercles are scattered about between these rows, 

 though without any definite arrangement; while they are abundant on the dorsal surfaces 

 of the two outer radials and of the large lower arm-joints (PL III. figs. 3-9), disappear- 

 ing, however, as the joints become more and more compressed laterally. 



All the entire specimens of Holopus which are known to science have been preserved 

 in the dry state, and have a blackish-green tint which is due to pentacrinin, as stated above 

 {ante, p. 129). It is darkest in the older individuals, and contributes to the shagreen-like 

 appearance that is so characteristic of the type. An isolated ray (PI. Va. fig. 3 ; 

 PI. Vb. figs. 4, 5) was, however, obtained by the " Blake " ofi" Montserrat, and preserved 

 in spirit. In this condition the skeleton has a dead white appearance. 



In the figure of d'Orbigny's original specimen the first radials are shown to bear large, 

 pentagonal axillaries which appear to be all in one piece. They were so described by 

 Pourtales,^ and also by Sir WyviUe Thomson,^ who did not, however, exclude the 

 possibility that they might be formed of the second and third radials coalesced, with the 

 syzygy between them obliterated. The very young individual dredged by the " Blake," 

 and the somewhat older form, shown in PL IV., throw much light upon this question. 

 The calyx-tube of the former (PL V. figs. 9, 10) is wide and shallow, while the second 

 radials which it supports are widely hexagonal and only partially in contact laterally. 

 Eesting upon their distal edges are the smaller triangular plates to which I have referred 

 as the axiUaries. Agassiz, in his brief description of this remarkable form,' states that the 

 larger hexagonal plates are "the radial axiUaries of Sir AVyville Thomson, but the smaller 

 triangular ones seem to become fused with them in the adult." I think, however, that 

 there can be little doubt that the larger plates are second radials, and the triangular ones 

 the third or axillary radials. They are all equal and similar, and meet one another all 

 round so as to completely close the cavity of the calyx. It might be suggested that 

 these are the combined second and axillary radials, while the hexagonal plates are the 

 first radials, and no others are present. I do not think, however, that this can be the 

 case ; partly on account of the very marked manner in which the hexagonal plates are 

 separated from one another and from the shaUow cup below them ; and partly because 

 there is no indication whatever of their sending upward extensions between the 



1 Mem. Mm. Comp. ZoUl, vol. iv. No. 8, p. 62, 1878. 



" Loc. cit., p. 408. 



^ Description of a young Holopus, Bull. Mus. Comp. Zoiil, vol. v. p. 213, 1879. 



