74 THE VOYAGE OF H.M.S. CHALLENGER. 
the interradial angles but not in the others. They are, however, very well developed in 
many of the fossil Neocrinoids, e.g., Marsupites, Apiocrinus, and Extracrinus; and 
they often have a very solid and substantial appearance. Like the anal plate they are 
most important in their palzontological relations ; as is also the single calyx-interradial of 
Thaumatocrinus (Pl. LVI. figs. 1-5), which has been already described (ante, pp. 39-41). 
The margins of the ambulacra of the disk, arms, and pinnules, and the interpalmar 
areas of the disk, are rarely, if ever, perfectly free from any traces of caleareous structures. 
Those of the ambulacra may take the form of simple short spicules which are almost 
entirely limited to the marginal leaflets ; or they may be forked and branching spicules, 
or rounded cribriform plates of variable size, which are movable and can either be 
erected or closed down over the grooves. They are well developed in Hyocrinus, 
Bathycrinus, and Rhizocrinus (Pl. Ve. figs. 8-10, ep; Pl. VI. figs. 1, 6; Pl. VIL 
he? cok, Vio digs, goo > abl Wallace. As PL hg PL x. die 20). in 
the first named genus they are sometimes separated from the dorsal skeleton by other 
plates which will be noticed later (Pl, Ve. figs. 9, 10, sp); and the same is often the case 
in the Pentacrinide and Comatulide (Pl. XXXIII. fig. 1; Pl. XLVII. figs. 11, 13; 
Pl XRTX. fies. 6, 7%; Pl. LL figs 11,12; Pl. LIL fes 5,6; Pl LIV. fies..416-9): 
These covering plates of the ambulacra of Pentacrinus were termed “ Saumplittchen ” 
by Miiller, to distinguish them from the uncalcified marginal leaflets of the ambulacra or 
“Saumlippchen,” which correspond to them in most Comatul ;* while Sars,’ doubting 
the mobility of these parts in Pentacrinus and Comatula, gave the name “Jamelles du 
sillon” to the large, oval, and movable plates which border the ambulacra of Rhizocrinus. 
All three structures, however, are of essentially the same nature. The covering plates of 
the stalked Crinoids are abundantly represented in many of the Comatule, always, 
however, resting upon a more or less developed “side plate” as in the Pentacrinide ; 
and the fact that they are merely an extensive development of the limestone rods and 
networks in the perisome bordering the ambulacra is evident when all the intermediate 
stages are examined. All the Pentacrinide have plated ambulacra; but this is by no 
means the case in the Comatulz, especially in Actinometra ; though individual species of 
Antedon have a relatively larger and more substantial ambulacral skeleton than any 
Pentacrinus or Metacrinus (compare Pl. XXVII. figs. 4, 11, 12; Pl. XXXIII. fig. 1; 
PI XUN IT figs. 11,13); PY LL figs. 11,12; PL LIL figs 5, 6 5 Pi abveses: 4 69). 
The covering plates which border the ambulacra on the arms and pinnules of 
Pentacrinus asteria were briefly mentioned by Miiller.2 But he described them as 
resting upon the joints of the skeleton, which is not always the case; and in fact, two 
pages further on he spoke of the plates which are situated on the perisome at the sides 
of the arms and pinnules, just as on that of the disk; while he does not seem to have 
? Ueber den Bau der Echinodermen, Abhandl. d. Berlin Akad., Jahrg. 1853, p. 57 (of separate copy). 
? Crinoides vivants, p. 24. 3 Bau des Pentacrinus, p. 46. 
