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



by Waclismutli and Springer on account of its resemblance to Eupacliyciinus. No sucli 

 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 Ensocrinus as in Encrinus, and since a ventral tube or sac like that of Ci/athocrinus 

 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 

 Erisocrimis. 



It should be remembered too that the Liassic Extracrinushas a symmetrical calyx with 

 a dicyclic base, i.e., of the same composition as that of Encrinus 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 Pateocrinoids, and do not therefore 

 believe that Erisocrinus must have had a closed vault because it was a Palseocrinoid. 

 Together with Encrinus, Philocri7ius, and Stemmatoci'inus it certainly aifords the best 

 transition yet known between the Neocrinoids and Palseocrinoids. For the only point of 

 difierence 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 Encrinus, as in most 

 other Neocrinoids. The occurrence of Encrinus-like forms in the Carboniferous strata of 

 India, America, Russia, and also Spain (according to C. Barrois) is therefore very interesting. 



Some species of Taxocrinus and Heterocrinus bave 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 Palseocrinoidea. 



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 Palseocrinoids, those of the Pentacrinidas are fully developed ; for it is 

 among the Comatulse that the greatest development of the arms is to be found. Very 

 few Pentacrinidse, except Extracrinus, 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 sjoecies 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 hundi-ed. Nearly all of these bear functional 

 pinnules, the last of which are sometimes longer than their predecessors. 



1 Revision, part i. p. 6. 



