154 THE VOYAGE OF H.M.S. CHALLENGER. 
by Wachsmuth and Springer on account of its resemblance to Hupachycrinus. No such 
vault has been found in this type, but only a small ‘‘ ventral tube” which rests on a series 
of three anal plates and has been traced to the height of the fourth or fifth arm-plate, where 
it is composed of small, very delicate, hexagonal plates. But these anal plates are absent 
in Hrisocrinus as in Encrinus, and since a ventral tube or sac like that of Cyathocrinus 
is always found associated with a system of anal plates, the lowest of which is inter- 
calated between two radials, it seems rash to postulate its presence in the symmetrical 
Erisocrinus. 
It should be remembered too that the Liassic Extracrinus has a symmetrical calyx with 
a dicyclic base, 7.e., of the same composition as that of Hnacrinus and Erisocrinus ; while 
its summit or ventral side was in no way different from that of a recent Pentacrinus. 
I have a strong suspicion that this is also true of many Paleeocrinoids, and do not therefore 
believe that Hrisocrinus must have had a closed vault because it was a Paleeocrinoid. 
Together with Encrinus, Philocrinus, and Stemmatocrinus it certainly affords the best 
transition yet known between the Neocrinoids and Paleocrinoids. For the only point of 
difference about which we are entitled to speak with certainty is the absence of a second 
radial in the three older forms, and the constancy of its presence in Hnerinus, as in most 
other Neocrinoids. The occurrence of Hnerinus-like forms in the Carboniferous strata of 
India, America, Russia, and also Spain (according to C. Barrois) is therefore very interesting. 
Some species of Zaxocrinus and Heterocrinus have been thought to bear a superficial 
resemblance to Pentacrinus, owing to the freedom of their rays. But both genera have 
an asymmetrical calyx with a well marked anal side, and also a variable number of radials, 
peculiarities which, when occurring together, are very characteristic of the Palzocrinoidea. 
Wachsmuth and Springer’ have pointed out that ‘another very characteristic distinc- 
tion between ancient and recent Crinoids is to be found in the comparatively large size 
and massive body plates in the fossil, contrasted with the diminutive body and very long 
and highly developed arms of recent types; and the same is even more strikingly true as 
to Blastoids and Cystideans.” 
But it is a mistake to suppose, as they do, that while the arms are in progress of 
growth in the Palocrinoids, those of the Pentacrinidee are fully developed; for it is 
among the Comatule that the greatest development of the arms is to be found. Very 
few Pentacrinide, except Hautracrinus, Pentacrinus asterius, and three or four species of 
Metacrinus have more than fifty arms, a number which is never reached by Apiocrinus 
and Bourgueticrinus. In certain species of Actinometra, however, the rays may branch 
six or eight times, and the number of arms exceed one hundred; while in many species 
both of Antedon and Actinometra, the number of joints in a single arm is over two 
hundred, and in rare cases reaches three hundred. Nearly all of these bear functional 
pinnules, the last of which are sometimes longer than their predecessors. 
1 Revision, part i. p. 6. 
