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 and 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 milUeri, its nearest ally, which has similar long and stout cirri ; while the 

 infra-nodal joint is usually somewhat grooved to receive the cirrus-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 mulleri ; 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 almo.st cubical or prismatic basal joints 

 of the first pinnules in Metacrinus (PI. XLIIl. figs. 2, 4). The preceding description 

 differs in one important respect from those given by Miiller and Lutken. 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 sef)arate the two joints, but 

 seems to have inferred that they were united by a bifascial articulation such as he found 

 m 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- Med usee.* This led Lutken to state ** that 

 the existence of an articulation between the two outer radials of Pentacrinus asterius was 

 an important character separating it from Pentacrinus tniilleri, 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 Pentacrin us asterius. In this respect, 

 therefore, this species resembles Pentacrinus mulleri, 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, pi. ii. fig. 5. - Bau des Penfcicriims, p. 43, Taf. ii., iii. 



^ Ibid., p. 26, Taf. ii. fig. 12. ■> Ibid., p. 30, Taf. ii. fig. 8. 



^ Om Vestindiens Pentacriner, loc. cit, p. 202. " Proc. Roy. Soc. Edin., vol. vii., 1872, p. "66. 



