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



ventral perisome of a recent Crinoid and the upper surface of the body beneath the vault 

 of an Actinocrimis. 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- 

 crinidfB. I believe this to be a true " ventral disk " similar to that of Pentacrimis, and 

 not in any way homologous to the soHd vault of the Actinocrinidaj. 



In describing the Palseocrinoids, 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, i.e., that in which the mouth is placed, with the 

 food-grooves radiating outwards from the peristomial area around it. Wachsmuth speaks 

 of this surface as the " soft or ventral peristome " (perisome 1), and says that it cannot in 

 the remotest degree be homologised with the solid vault of the PaliBocrinoids. Hence his 

 occasional 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 Actinocrimis. This surface corresponds to the ventral 

 disk of Pe«tacrmMs; and " interpalmar " is here used by Wachsmuth in the same sense 

 as it was by Mtiller, i.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 structm-es. The former 

 is necessarily formed of closely-fitting solid plates ; while the latter, lying beneath it, 

 may be bare as in many Comatulse, or more or less completely plated as in Actinocrinus 

 and the Pentacrinidse. But no recent Crinoid, not even Tliaumatocrinus, has anj'thing 

 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 XVH. 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 Actinocrinidte and Platycrinidte ; though Haplo- 



1 Amer. Journ. Sci. and Arts, vol. xiv. pp. 181, 185. Revision, part i. pp. 5, 32, 54 ; part ii. p. 53. 

 - Revision, part ii. pp. 26, 27, 31. ' Revision, part ii. pp. 64, 107. 



