REPORT ON THE CRINOIDEA. 5 
(Pl. XXVI. figs. 5,8; Pl. XX XVII. fies. 3, 4; Pl. L. figs. 6, 7, 12, 13) like the apposed 
surfaces of the radials and basals respectively (Pl. XX. figs. 2, 3, 6, 9). This is also the 
case in Rhizocrinus, which presents another peculiarity as well. Near the dorsal edge of 
the upper face of the hypozygal there is a more or less well-marked pit (Pl. X. figs. 1, 6, 
8, 18), and a corresponding peg-like process projects from the under face of the epizygal 
(Pl. X. figs. 17, 19), so that closeness of union is effected in this way instead of by the 
usual radiating ridges. 
Another arrangement which effects a somewhat closer syzygial union than usual, 
presents itself in the arms of Pentacrinus naresianus (Pl. XXX. figs. 20, 21, 23; 
Pl. XXXa. figs. 9, 10, 12) and of Pentacrinus blakei (P). XXXII. figs. 4, 5, 7,9, 12, 14). 
The apposed surfaces are not flat or slightly curved, but the proximal face of the epizygal 
rises to a sharp crest which is interrupted by the central canal, and fits into a correspond- 
ing re-entering angle on the distal face of the hypozygal, so that the two joints interlock 
very closely. This peculiarity is very apparent in a side view of the arm (Pl. XXX. 
fic. 23) ; but when seen from the dorsal side, the distal edge of the hypozygal appears to 
be very convex and to project strongly forward into the epizygal (Pl. XXX. fig. 1; 
Pl, XXXI. fig. 2). 
Very different modes of articulation occur in the Crinoid skeleton, so that the amount 
of play between two successive joints varies considerably. It is probably at a minimum 
in the stem of Hyocrinus (Pl. VI. fig. 2). We do not know the nature of the basal part of 
the stem; but the lower joints of the fragment, 170 mm. in length, which was obtamed, 
are cylindrical, with their terminal faces devoid of any fossx, but marked by a radiating 
pattern of grooves and ridges. This, however, is sometimes absent, as shown in 
Pl. Ve. fig. 4. Sir Wyville Thomson spoke of the joints as united by a close syzygial 
suture; but the ligamentous fibres which effect their union (Pl. Ve. fig. 5,/s) are 
longer than I have ever seen them in any real syzygy, and rather resemble those which 
unite the successive pinnule joints of other Crinoids when muscles are absent. They are 
all of the same length and not longer in the centre than at the periphery, as they are 
between the deeply hollowed stem-joints of the Bourgueticrinide (Pl. VIIa. figs. 8-11 ; 
Pl. X. figs. 11-14). But I should hardly describe this mode of union as a syzygy 
(or suture); for there must have been some amount of play between the successive 
joints, and a syzygy was described by Miiller as an immovable sutural union of two 
joints.” 
Next to Hyocrinus, the Pentacrinides have the most closely united stem-joints. In 
this family each internode of the stem contains five oval bands of elastic fibres (Pl. XXIV. 
figs. 1, 3-5, ds). They run through all the joints between the hypozygal of one syzygy 
and the epizygal of the next belowit, which isthe true nodal joint. Miiller® and Wyville 
1 Notice of new living Crinoids belonging to the Apiocrinidz, Jowrn. Linn. Soe. Lond. (Zool.), vol. xiit. p. 52, 1876, 
2 Op. cit., p. 39. 3 Op. cit. p. 17. 
