304 THE VOYAGE OF H.M.S. CHALLENGER. 
type but the slender Pentacrinus decorus. But as in so many other species it was liable 
to fracture just below a node, so that the individual led a semi-independent existence; for 
I have one stem-fragment in which the lowest joint is decidedly worn aud its central canal 
closed up by a small round boss which projects above the remaining surface of the joint. 
The length of the internodes in Pentacrinus asterius distinguishes it at once from 
Pentacrinus miilleri, its nearest ally, which has similar long and stout cirri; while the 
infra-nodal joint is usually somewhat grooved to receive the eirrus-bases. This is but 
rarely the case in Pentacrinus asterius, and then only to the slightest possible extent, so 
that the cirrus-socket is practically limited to the articular facet, without any extension 
either upwards or downwards. The stem of Pentacrinus asterius is thus very readily 
identified, and the same may be said of the arms, which is rarely the case with the other 
species of the genus, unless the ventral groove be examined. For the peculiar features of 
the pinnules are very characteristic. They are well shown in Miller's figure’ and likewise 
in those given by Miiller,? who specially referred to the projections from the ends of the 
pinnule-joints. The great thickness of the basal joints in the distichal and palmar 
pinnules, especially the former, is a somewhat unusual character in a Pentacrinus. A trace 
of it may be seen in some forms of Pentacrinus miilleri ; but in most species of the genus 
the lower joints of the first pinnules are laterally compressed, and lie close against the 
arm. They thus present a great contrast to the almost cubical or prismatic basal joints 
of the first pinnules in Metacrinus (Pl. XLIII. figs. 2, 4). The preceding description 
differs in one important respect from those given by Miiller and Liitken. The former 
author spoke of the union between the second and third radials as an articulation, but 
was somewhat obscure about its nature. He was not able to separate the two joints, but 
seems to have inferred that they were united by a bifascial articulation such as he found 
in Antedon rosacea.’ While, however, he stated expressly that the latter type has no 
muscles between the second and axillary radials, he described and figured muscles as 
existing in this position in Pentacrinus caput-Meduse.* This led Liitken to state® that 
the existence of an articulation between the two outer radials of Pentacrinus asterius was 
an important character separating it from Pentacrinus miilleri, which has these joints united 
by syzygy. Neither he nor Miiller, however, had ever actually separated the joints, 
specimens being then too valuable, or the real state of the case would have become evident 
at once. This was first discovered by Sir Wyville Thomson,° who pointed out that there 
is really a syzygy between the two outer radials of Pentacrinus asterius. In this respect, 
therefore, this species resembles Pentacrinus miilleri, instead of differing from it as was 
supposed by Liitken. But unfortunately Sir Wyville totally misconceived the real 
character of Oersted’s species, and confounded it with the one previously described by 
} Op. cit., p. 51, pl. ii. fig. 5. ? Bau des Pentacrinus, p. 48, Taf. ii., iii. 
% Ibid., p. 26, Taf. ii. fig. 12. 4 Ind., p. 30, Taf. ii. fig. 8. 
° Om Vestindiens Pentacriner, loc. cit, p. 202. ® Proc. Roy. Soc. Edin., vol. vii., 1872, p. 766. 
