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



reduced aud partially resorbed conditiou.' In Bhizocrinus, Hyocrinus, Holopus, and 

 Thaumatocrimis they persist through life, and in each case present a different stage of 

 development. 



The orals of Holojms retain their embryonic position, and are scarcely separated at all 

 from the first radials, coming into close relation with the inner faces of these plates, while 

 the arms are altogether above and outside them (PL III. fig. 2). But in Hyocrimis 

 (PL Vc. fig. 6, O; PL VI. figs. 1-4) and also in Thaumatocnnm (PL LVI. fig. 5), 

 though still relatively large, they are separated from the edges of the radials by a 

 marginal zone of perisome which is paved with closely-set plates, and occupies about one- 

 fifth of the total diameter of the disk. 



The orals appear to be unequally developed in the two living species of Rhizocrinus. 

 In Rhizocrinus lofotensis they would seem either to undergo some amount of resorption, 

 or else to remain in a comparatively undeveloped condition. For they are figured and 

 described by Sars ^ as minute valvule-like plates which occupy the central ends of the 

 triangular interpalmar fields on the disk ; so that there is a comparatively large amount 

 of perisome between their bases and the edge of the disk, just as there is in many young 

 Comatulje after separation from the stem. In Rhizocrinus raivsoni, however, they are 

 relatively larger, and their bases approach more closely to the lower brachials, from which 

 they are only separated by a narrow band of perisome (PL X. figs. 7, 20). 



Under these circumstances, therefore, it is hardly to be expected that the orals 

 should 1)0 preserved in the fossil species of Rhizocrinus ; for as they are only united 

 to the calyx by membrane, they would naturally become separated from it when the soft 

 parts were destroyed. In all the recent Comatulse, with the exception of the archaic 

 tj-pe Tliaumatocrinus (PL LVI. fig. 5), they are ]-esorbed before maturity is reached ; 

 and if this was not the case in the fossil species, they probably persisted in somewhat 

 the same form as in Rhizocrinus. Even in Holopus there is nfi very close connection 

 between the orals and the tubular cup (PI. III. figs. 1,2); and the type is so rare in the 

 fossil state, that specimens with the orals preserved are not likely to be found. 



In the Palseocrinoids, however, the orals reached a greater development than in the 

 later Neocrinoids, resembling rather the solid plates of Holopus and Hyociinus (PL III. 

 fig. 2 ; PL Vc. fig. () ; PL VI. figs. 1-5) than the mere films of delicate limestone 

 network which represent them in Rhizocrinus and in the Comatulfe. It will, however, be 

 more advantageous to postpone the discussion of the nature and position of the oral 

 plates in the Palasocrinoids until the chapter which deals with the relation of these older 

 forms to the Neocrinoids. 



1 Fra den norske Nordhavs-E.xiitditioii, Echinodei-mer, Nyt Mag. f. Naturvid., Bd. xxiii. ]\ '.). 

 - Crinoides vivnnts, p. 17, 1'gs. 40, 41, 85, s(i. >^9-91— ». 



