REPORT ON THE CRINOIDEA. 5 



(PI. XXV l. figs. 5, 8 ; PL XXXVII. figs. 3, 4 ; PI. L. figs. 6, 7, 12, 13) like the apposed 

 surfaces of the radials and basals respectively (PI. 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 (PI. X. figs. 1, 6, 

 8, 18), and a corresponding peg-like process projects from the under face of the epizygal 

 (PI. 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 oi Pentacrinus Uakei (PL 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 hy|Jozygal, so that the two joints interlock 

 very closely. This peculiarity is very apparent in a side view of the arm (PL XXX. 

 fig. 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 ; 

 PI. 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 Hyocrvrms (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 obtained, 

 are cylindrical, with their terminal faces devoid of any fossaj, but marked by a radiating 

 pattern of grooves and ridges. This, however, is sometimes absent, as shown in 

 PL Vc. 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 Vc. fig. 5, Is) 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 w^hen 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 Bourgueticrinidse (PL Vila. 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 Hijocrinus, the Pentacrinidai 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, Is). They run through all the joints between the hypozygal of one syzygy 

 and the epizygal of the next below it, which is the true nodal joint. Miiller^ and Wyville 



1 Notice of new living Crinoids belonging to the Apiocrinidii', Journ. Linn. Soc. Land. (Zool.), vol. xiii. p. 52, 1S76, 



2 Oji. cit., p. 39. ^ Op. cif., p. 17. 



