fe THE VOYAGE OF H.M.S. CHALLENGER. 
reduced and partially resorbed condition.’ In Rhizocrinus, Hyocrinus, Holopus, and 
Thaumatocrinus they persist through life, and in each case present a different stage of 
development. 
The orals of Holopus 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 (PI. III. fig. 2). But in Hyocrinus 
(Pl. Ve. fig. 6, O; Pl. VI. figs. 1-4) and also in Thaumatocrinus (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 Fhizocrinus 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 
Comatule after separation from the stem. In Rhizocrinus rawsoni, 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 (PI. X. figs. 7, 20). 
Under these circumstances, therefore, it is hardly to be expected that the orals 
should be 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 Comatulz, with the exception of the archaic 
type Thaumatocrinus (Pl. LVI. fig. 5), they are resorbed 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 no very close connection 
between the orals and the tubular cup (Pl. 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 Paleocrinoids, however, the orals reached a greater development than in the 
later Neocrinoids, resembling rather the solid plates of Holopus and Hyocrinus (PI. III. 
fig. 2; Pl. Ve. fig. 6; Pl. VI. figs. 1-5) than the mere films of delicate limestone 
network which represent them in Rhizocrinus and in the Comatule. It will, however, be 
more advantageous to postpone the discussion of the nature and position of the oral 
plates in the Paleocrinoids until the chapter which deals with the relation of these older 
forms to the Neocrinoids. 
1 Fra den norske Nordhays-Expedition, Echinodermer, Nyt Mag. f. Naturvid., Bd. xxiii. p. 9. 
2 Crinoides vivants, p. 17, figs. 40, 41, 85, 86, 89-91—o. 
