176 THE VOYAGE OF H.M.S. CHALLENGER. 
narrow ridges, which bifurcate twice within the body." Interradial dome plates larger than 
the radial, apical plates not prominent and identified with difficulty, 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 
plates 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 (Pl. XVII. fig. 6; Pl. LIV. fig. 10; Pl LV. figs. 4, 5, 7). 
From the numerous bifureating ridges formed by the radial dome plates, I cannot help 
suspecting that these plates are not true vault pieces as in the Actinocrinide, but the 
covering plates of closed ambulacral tunnels. They have a very different arrangement 
from the various series of radials on the abactinal side, which should not be the case if 
they belong to the vault, ze¢., 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 Marsupiocrinus. 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 difference between it and the plated disk of Pentacrinus wyville-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 Marswpiocrinus 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 Actinocrimide. In the recent Hyo- 
crinus, Which has many Paleeocrinoid affinities, the food-grooves pass from the oral to 
the ambulacral system in the body before they reach the arms (PI. VI. figs. 1-4); and 
I see no reason why they should not have done the same in some of the Platycrinide, 
the family which is supposed by Wachsmuth to represent an incompletely developed 
condition of the Actinocrinide. 
In Platycrinus, Hexacrinus, and Talarocrinus the structure of the vault is 
1 They bifurcate considerably more than “twice ” in Marsupiocrinus radiatus. 
2 Op. cit., tab. x. figs. 16, 21. 
