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 Hyocrinus the third and following joints are triple and not double 
only. A similar difference between the arms of Heterocrinus simplex and Heterocrinus 
constrictus has been already noticed." 
The arrangement of the pinnules of Hyocrinus was described by Sir Wyville Thomson 
s “hitherto entirely unknown in recent Crinoids, although we have something very close 
to it in some species of the Paleozoic genera Poteriocrinus and Cyathocrinus.”* 1 do 
not think, however, that this resemblance is such a very close one after all. For the 
lateral appendages of the arms of Hyocrinus, 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 arms 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 remain perfectly simple throughout their whole length, great though 
this may be. It has been already pointed out that the nearest approach to the 
pinnule arrangement of Hyocrinus is to be found in Barycrinus herculeus 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 bear no pinnules, so that they seem to correspond somewhat closely 
with the pinnules of Hyocrinus. ; 
The closest approximation among the Necuenaids to the arrangement of the pinnules 
which occurs in Hyocrinus, though still differing from it in important points, seems to 
me to be found in the Liassic genus Hxtracrinus. 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 
original arm-trunk and of its numerous armlets are all on about the same level. In this 
respect the armlets of Hxtracrinus 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 Hyocrinus is specially related to any of the 
other Neocrinoidea, while it presents important characters which connect it with the 
Paleeocrinoids. 
1 Ante, p. 53. 2 Journ. Linn. Soc. Lond. (Zool.), vol. xiii. p. 52. 
