218 



THE VOYAGE OF H.M.S. CHALLENGER. 



by syzygy into groups of two or three, only the terminal joints 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. JMouth 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 (PI. Vc. figs. 4-10; PI. VL). 



Hyocrimtshethellianus, Wyv. Thorns., Journ. Linn. Soc. Load. (Zool.) (1876), 1878, vol. xiii. p. 51. 

 Hyocrimts hethellianus, Wyv. Thorns., The Atlantic, 1877, vol. ii. pp. 96-99. 



Dimensions. 



Total length of calyx and arms {fide C. W. T. ), 



Total height of calyx. 



Total diameter of calyx, 



Height of radial, 



Width of radial, 



Height of oral plates, 



Length of first pinnule. 



Diameter of arm-joints. 



Greatest height of stem-joints. 



Diameter of stem-joints, 



60-00 mm. 



7-25 



6-00 



4-00 



3-00 



2-75 

 31-00 



1-00 



1-15 



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 (PI. Vc. fig. 5, Is). The 

 terminal faces of the joints (PI. Vc. fig. 4) are slightly hollowed, and either plain or 

 marked with indistinct radiating striae. 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 

 (PI. Vc. 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 (PI. 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 PI. 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 Palaeocrinoids. The 

 small single basal is the one immediately to the right of the anus, i.e., in the interradius 

 beyond the anus to a watch-hand, when the disk is placed upwai'ds. The remainder of 



