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



Criuoid. Did it appear, it would only be in the way, and have to undergo resorption to 

 a greater or less extent, just as the dorsocentral of many Urchins is more or less com- 

 pletely resorbed after the appearance of the anus. 



As regards the recent Crinoids, therefore, the embryological evidence clearly indicates 

 that the basals of the abactinal system are represented in the actinal system by the orals. 

 The former are within the ring of radials and next to the dorsocentral ; and it seems 

 therefore only natural to regard the six proximal interradial plates surrounding the 

 central piece (orocentral) in the vault of a Palaeocrinoid as representing oral plates. 



"Wachsmuth admits that Zittel was right in regarding the interradial pla.tes which 

 form the dome of Hcqjlocrinus as representing the orals of Neocrinoids ; and he takes 

 the same view of the interradial plates in the dome of Symhathocrimis. I should have 

 thought therefore that he would have given a similar interpretation of the summit plates 

 in Platycrinus and Cidicocrinus. Those of the latter genus were described by Miiller as 

 follows : — " Der Scheitel besteiit aus 5 Tafeln, welche dicht an einander schliessen durch 

 Niihte und anderseits bis an die Arme und die Interradialia reichen, sie stehen iibrigens 

 interradial, so dass jedes Interradiale ^u einem der funf Scheitelstiicke stimmt. Der 

 Mund {cf. anus) befindet sich seitlich in einem Interradius zwischen dem Interradiale 

 und entsprechenden Scheitelstuck. Auf jeder der 5 Scheiteltafeln ea-hebt sich ein 

 kurzer Dorn."^ Wachsmuth calls these " Scheitelstiicke " of MilUer's the proximal vault 

 pieces, and suggests that " probably his largest plate includes four plates, the spiniferous 

 central vault piece, the two proximal vault pieces, and a small anal plate between them. 

 The four large proximal vault pieces, each crowned with a spine, are no doubt correctly 

 represented. Those few plates occupy the greater part of the summit, leaving Ijut little 

 space for the radial dome plates, which as yet are unknown." ^ 



I think myself that this suggestion is totally unnecessary ; for the five summit plates 

 appear to me to form a closed oral pyramid :in which the plate on the anal side is somewhat 

 wider than its fellows, but not divided into two as in the Actinocrinidas and Platycrinidse. 



CuUcocrinus has the proximal dome plates resting against the calyx interradials, just 

 as the orals do in Coccocrinus. But no distal ring of radial dome plates is known in 

 this genus. This advance in complexity is presented by certain forms of Platycrinus from 

 the Carboniferous limestone, both at Burlington and at Bolland. Miiller gave some 

 figures of the latter under the name of Platycrinus ventrieosus, Goldfuss, which is 

 certainly wrong ;^ and I have examined several specimens in the British Museum collec- 

 tion which are in this comparatively primitive condition. The proximal dome plates rest 

 directly against the calyx interradials,* that on the posterior side being represented by 

 two small plates with the anus between them ; while there is a more or less tubercular 



^ Verlil. d. Naturhist. Verein. d. preuss. Eheinl., Bd. xii., Jahrg. 1855, p. 23. 



' Revision, part ii. p. 62. ^ Bau des Pentacrinus, loc. cit., Taf. vi. figs. 1«, 16. 



* It is only just to Mr. AVaclismuth for me to state here that he was the tir.st to direct my attention to the fact that 

 Platycrinidie e.\ist with tlie proximal dome plates resting directly against the calyx interradials, two individuals in this 



