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., Ilarsupites, Ajnocrinus, and Extracrinus ; and 

 they often have a very solid and substantial appearance. Like the anal plate they are 

 most important in their palseontological relations ; as is also the single calyx-interradial of 

 Tliaumatocrinus {J*l. LVI. figs. 1-5), which has been already described {ante, pp. 39-41). 



The margins of the ambulacra of the disk, anns, and pinnules, and the interpaknar 

 areas of the disk, are rai'ely, if ever, perfectly free from any traces of calcareous structures. 

 Those of the ambidacra 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 (PI. Vc. figs. 8-10, cp; PI. VI. figs. 1, 6 ; PL VII. 

 fig. 7 ; PL VIIL figs. 3, 5 ; PL Villa, fig. 1 ; PL IX. fig. 4 ; PL X. fig. 20). In 

 the first named genus they are sometimes separated from the dorsal skeleton by other 

 plates which will be noticed later (PL Vc. figs. 9, 10, sp) ; and the same is often the case 

 in the Pentacrinidaj and ComatuHdiB (PL XXXIII. fig. 1; PL XLVII. figs. 11, 13; 

 PL XLIX. figs. 6, 7 ; PL LI. figs. 11, 12 ; PL LII. figs. 5, 6 ; PL LIV. figs. 4, 6-9). 



These covering plates of the ambulacra of Pentacrinus were termed " Saumpliittchen " 

 by MliUer, to distinguish them from the uncalcified marginal leaflets of the ambulacra or 

 " Saumliippchen," which correspond to them in most Comatulas ;^ while Sars,^ doubting 

 the mobility of these parts in Pentacrinus and Comatula, gave the name " lamelles 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 Comatulje, always, 

 however, resting upon a more or less developed " side plate " as in the Pentacriuidce ; 

 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 Pentacrinidte have plated ambulacra ; but this is by no 

 means the case in the Comatulse, 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; 

 PL XLVII. figs. 11, 13 ; PL LI. figs. 11, 12 ; PL LIL figs. 5, 6 ; PL LIV. figs. 4, 6-9). 



The covering plates which border the ambulacra on the arms and pinnules of 

 Pentacrinus asteria were briefly mentioned by JMliUer.' 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 j)erisome at the sides 

 of the arms and pinnules, just as on that of the disk ; while he does not seem to have 



' Ueber deu Bau der Echinodermen, Ahluindl. d. Berlin A':ad., Jakrg. 1853, p. 57 (of separate copy). 

 ^ Crinoides vivants, p. 24. ^ Bau dea Pentacriuus, p. 46. 



