166 THE VOYAGE OF H.M.S8. CHALLENGER. 
ventral perisome of a recent Crinoid and the upper surface of the body beneath the vault 
of an Actinocrinus. Both had plated ambulacra and anambulacral plates. The admitted 
resemblance of the interpalmar anambulacral plates of the Actinocrinite to ‘‘ vault pieces ” 
is another point in favour of the view which I take of the so-called vault in the Ichthyo- 
crinide. I believe this to be a true “ ventral disk” similar to that of Pentacrinus, and 
not in any way homologous to the solid vault of the Actinocrinide. 
In describing the Palzocrinoids, Wachsmuth uses “ vault,” “summit,” and “ ventral 
disk” as synonymous terms.’ This is somewhat unfortunate, as tending to lead to con- 
fusion. 
The expression “ oral disk” or “ ventral disk” is universally used to denote the upper 
surface of the visceral mass of a Crinoid, 7.e., that in which the mouth is placed, with the 
food-grooves radiating outwards from the peristomial area around it. Wachsmuth speaks 
’ (perisome ?), and says that it cannot in 
the remotest degree be homologised with the solid vault ofthe Paleeocrinoids. Hence his 
oceasional reference to this vault as a ventral disk is a little confusing ; and his use of the 
word “interpalmar” is equally so. 
He sometimes employs it’ to denote the interradial spaces between the ambulacral ridges 
on the upper surface of the casts of Actinocrinus. This surface corresponds to the ventral 
disk of Pentacrinus ; and “interpalmar” is here used by Wachsmuth in the same sense 
as it was by Miiller, z.e., for the “interradialen Felder zwischen den Tentakelrinnen.” 
When therefore he employs “interpalmar” to denote the interradial plates of the vault,° 
its meaning is entirely different; for the vault was a dome of solid plates, completely 
concealing the mouth, food-grooves, and interpalmar areas on the ventral disk. 
The vault and ventral disk are, to my mind, entirely distinct structures. The former 
is necessarily formed of closely-fitting solid plates; while the latter, lying beneath it, 
of this surface as the “ soft or ventral peristome’ 
may be bare as in many Comatule, or more or less completely plated as in Actinocrinus 
and the Pentacrinide. But no recent Crinoid, not even Thawmatocrinus, has anything 
like a dome or vault rising above a ventral disk. Numerous specimens are known with 
the covering plates at the sides of the food-grooves closed over them so as to convert them 
into tunnels (Pl. XVII. fig. 6; Pl. LV. figs. 3-7). But this was also the case beneath the 
vault of Actinocrinus. Sometimes, indeed, the plates of the disk may be so closely set that 
the opening of the mouth, which may be large in other specimens, is nearly or quite con- 
cealed, as in the so-called recent Cystidean Hyponome (Pl. LIV. fig. 10; PL LV. 
figs. 4, 5, 7). But this is in no way comparable to the embryonic closure of the mouth 
before the separation of the valves of the oral pyramid. It is only in this and similar 
cases that I admit the presence of a vault, dome, or tegmen calycis. This structure 
reaches its fullest development in the Actinocrinide and Platycrinide ; though Haplo- 
1 Amer. Journ. Sct. and Arts, vol. xiv. pp. 181, 185. Revision, part i. pp. 5, 32, 54; part 11. p. 53. 
* Revision, part ii. pp. 26, 27, 31. 3 Revision, part ii. pp. 64, 107. 
