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



narrow ridges, which bifurcate fwice within the body.^ Interradial dome plates larger than 

 the radial, apical plates not prominent and identified with difiiculty, interpalmar spaces 

 paved with small pieces." Figures of the vault of Marsupiocrinus radiatus and 

 Marsupiocrinus depressus are given by Angelin.^ The former shows a few larger plates 

 in the centre which may be the apical dome plates ; but in the other figure none of the 

 jjlates in the centre are specially large ; so that it is possible that Marsupiocrinus may 

 resemble many recent Crinoids in the total resorption of the orals, causing the proximal 

 ends of the interpalmar areas to be thickly studded with plates which tend to obscure 

 the position of the mouth (PI. XVII. fig. 6 ; PL LIV. fig. 10 ; PI. LV. figs. 4, 5, 7). 

 From the numerous bifurcating ridges formed by the radial dome plates, I cannot help 

 suspecting that these plates are not true vault pieces as in the Actinocrinidse, but the 

 covering plates of closed ambulacral tunnels. They have a very diflerent arrangement 

 from the various series of radials on the abactinal side, which should not be the case if 

 they belong to the vault, i.e., to the oral system. It will be remembered that 

 Wachsmuth has compared them to covering plates, while regarding them as true vault 

 pieces ; and he speaks of the interradial areas between them as " interpalmar," a term 

 which is inapplicable to true vault pieces, though I think he has used it correctly in the 

 case of Marsupioc7'inus. For I have a very strong impression that the so-called vault of 

 this genus is really the strongly plated ventral perisome, in the centre of which the 

 remains of the orals (apical dome plates) are perhaps to be found. I cannot see any 

 such essential diff'erence between it and the plated disk of Pentacrinus ivyville-thomsoni 

 or of many Antedons (PL XVII. fig. 6 ; PL LV.) as would lead to the supposition that 

 the homologue of the latter is to be sought for beneath the vault of Marsupiocrinus. At 

 the same time I have no intention of asserting the presence of an external mouth and 

 open food-grooves on the calyx of this genus. For although these are present in 

 the apparently similar disks of the recent forms, I think it not impossible that the 

 tentacular vestibule over the peristome of Marsupiocrinus may never have opened to the 

 exterior, and that the covering plates of the food-grooves proceeding from it may have 

 been immovably closed down over them. They were thus converted into tunnels, but 

 were still " external," in the sense of not being covered by a " tegmen," as those were which 

 formed the tubular skeleton beneath the vault of the Actiuocrinidte. In the recent Hyo- 

 crinus, which has many Palneocrinoid afiinities, the food-grooves pass from the oral to 

 the ambulacral system in the body before they reach the arms (PL VI. figs. 1-4) ; and 

 I see no reason why they should not have done the same in some of the Platycrinidse, 

 the family which is supposed by Wachsmuth to represent an incompletely developed 

 condition of the Actinocrinidse. 



In Platycrinus, Ilexacrinus, and Talaivcrinus the structure of the vault is 



1 They bifurcate considerably more than " twice " in Marsupiocrinus radiatus. 



2 Op. cii., tab. x. tigs. IG, 21. 



