218 THE VOYAGE OF H.M.S. CHALLENGER. 
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by syzygy into groups of two or three, only the terminal jomts of which bear pinnules. 
Lowest pinnules the longest, and the following ones proportionately shorter, so that they 
all terminate on the same level as the arm-ends. Mouth protected by five large oral 
plates. Stem composed of short, cylindrical joints with simple or slightly striated faces. 
Mode of attachment unknown. 
A. GENERAL AccouNT OF THE TYPE. 
Hyocrinus bethellianus, Wyville Thomson, 1876 (Pl. Ve. figs. 4-10; Pl. VI). 
Hyoerinus bethellianus, Wyv. Thoms., Journ. Linn. Soc. Lond. (Zool.) (1876), 1878, vol. xiii. p. 51. 
Hyocrinus bethellianus, Wyv. Thoms., The Atlantic, 1877, vol. ii. pp. 96-99. 
Dimensions. 
Total length of calyx and arms ( ‘ie C. W. ©. a ; ; : - 60:00 mm. 
Total height of calyx, ; ‘ 5 F : T4255 3 
Total diameter of calyx, 5 5 : 3 : ; : 6:00) =; 
Height of radial, F ; : : : d : : 4:00 ,, 
Width of radial, ‘ : : : é : : : 3:00), 
Height of oral plates, : : ; , ; : . PETG) 
Length of first pinnule, : : : ; 3 . eos OO lar 
Diameter of arm-joints, C . - : : : : L007, 
Greatest height of stem-joints, ; : ; : ; : ist) a5 
Diameter of stem-joints, : ; ; ; : ; . 1:25 ,, 
The stem is rigid, and consists of short, cylindrical joints, usually a trifle higher than 
wide, and closely united by thin disks of ligamentous fibres (Pl. Ve. fig. 5, Js). The 
terminal faces of the joints (Pl. Ve. fig. 4) are slightly hollowed, and either plain or 
marked with indistinet radiating striez. The opening of the central canal is more or less 
definitely stellate, and in the substance of each joint itself there is a considerable space 
(Pl. Ve. fig. 5, rs) around the central axis (ca). The longest portion of the stem obtained 
was about 170 mm. in length, but its mode of attachment is not known. Towards the 
upper end the joints become much shorter, and in the uppermost 5 mm. they are mere 
disks with a slightly increased width (Pl. VI. fig. 3). The cup, which enlarges gradually 
upwards, consists of two tiers of very thin plates, the basals and radials, the latter being 
rather the higher of the two. The basiradial and the five interradial sutures are fairly 
distinct, but neither in the specimen represented in Pl. VI. nor in a fragment from the 
same locality, can I make out more than three interbasal sutures. Were it not that 
this seems to be the case in both specimens, I should be inclined to regard it as unim- 
portant; but under the circumstances I think we must consider that the lower part of 
the cup consists of two larger pieces and one smaller one, as in.certain Palocrinoids. The 
small single basal is the one immediately to the right of the anus, ¢.e., in the interradius 
beyond the anus to a watch-hand, when the disk is placed upwards. The remainder of 
