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



It is in the character of the arms and pinnules, however, that Hyocrinus is most 

 remarkable. The syzygial union of successive pairs of arm-joints is characteristic of 

 Rhizocrinus ; but in Hyoci'inus the third and following joints are triple and not double 

 only. A similar difference between the arms of Heterocrinus simplex and Heterocrinus 

 C07istrictus has been already noticed.^ 



The arrangement of the pinnules of Hyocrinus was described by Sir Wyville Thomson 

 as " hitherto entirely unknown in recent Crinoids, although we have something very close 

 to it in some species of the Palaeozoic genera Poteriocrinus and Cyatliocrinus"^ I do 

 not think, however, that this resemblance is such a very close one after all. For the 

 lateral appendages of the arms of Hyocrimis, long as they may be, are true pinnules. 

 Cyathocrinus, on the other hand, has no pinnules whatever, but long branching arms, 

 each branch bifurcating several times. It is true that the terminations of all the branches 

 are about on the same level, as is the case with the aims and pinnules of Hyocrinus. 

 But in the one genus a bifurcation gives rise to two equal arms which divide again, 

 and in the other there is no bifurcation at all, but the arm-joints bear a series of 

 pinnules which i-emain perfectly simple throughout their whole length, great though 

 this may be. It has been ah-eady pointed out that the nearest approach to the 

 pinnule arrangement of Hyocrimis is to be found in Barycrinus herculeiis from 

 the Carboniferous series of Indiana, United States {ante, p. 61). The so-called 

 armlets of this type alternate with one another upon opposite sides of the main 

 arm-trunk and l^ear no pinnules, so that they seem to correspond somewhat closely 

 with the pinnules of Hyocriiiits. 



The closest approximation among the Neocrinoids to the arrangement of the pinnules 

 which occurs in Hyocrinus, though stiU differing from it in important points, seems to 

 me to be found in the Liassic genus Uxtracnnus. In this curious type each arm 

 consists of a principal trunk bearing pinnules as usual, and giving off at intervals from 

 its inner side a series of smaller armlets which also bear pinnules. The lowest of these 

 are as long as the remaining portion of the arm -trunk from which they spring; and 

 the following ones are of successively diminishing lengths, so that the ends of the 

 orio-inal arm-trunk and of its numerous armlets are all on about the same level. In this 

 respect the armlets of Extrcicrinus are comparable to the pinnules of Hyocrinus ; but 

 they bear pinnules themselves, and only come off from one side of the main arm-trunk, 

 instead of alternating from opposite sides.. 



There is, therefore, no exact parallel to the condition of the arms of Hyocrinus 

 to be found in any Neocrinoid; and remembering this, as well as the peculiarities 

 of the calyx, we cannot say that Hjocrinus is specially related to any of the 

 other Neocrinoidea, while it presents important characters which connect it with the 

 Palaeocrinoids. 



' Ante, p. 53. ^ Journ. Linn. Soc. Loud. (Zool.), vol. xiii. jr. 52. 



