14 THE VOYAGE OF H.M.S. CHALLENGER. 
and there is no more direct communication between the fibres forming one of the tendons 
that ends in a nodal joint and the fibres of the syzygy between that joint and the 
infra-nodal, than there is between the ligaments uniting the second radials of Pentacrinus 
wyville-thomsoni to the first (PL XXIV. figs. 8, 9, 7d), and the fibrils of the syzygy 
between the second and third radials. . In both cases the two sets of fibres are separated 
by the organic basis of a joint of the skeleton; in the one of the ray, and in the other of 
the stem. 
The amount of increase in the size of the nodal joint varies considerably in the 
different species. They are not specially prominent in Pentacrinus asteria (Pl. XIII. 
figs. 4, 8), Pentacrinus miilleri (Pl. XV. fig. 4), Pentacrinus blakei (Pl. XXXI. fig. 3), 
or in Pentacrinus naresianus (Pl. XXVIII. fig. 2; Pl. XXXa. fig. 6). In Pentacrinus 
decorus, on the other hand (Pl. XXXVI.), the joint expands considerably from its upper 
edge down to the top of each cirrus-socket, and then narrows again ; while in Pentacrinus 
wyville-thomsoni (Pl. XIX. figs. 3, 4) the sockets are very prominent, and the joint is 
widest near its lower edge. In the genus Metacrinus, however, the cirrus-sockets are by 
no means specially prominent (Pl. XXXIX. fig. 3; Pl. XLI. figs. 1, 5,15; Pl. XLVI. 
figs. 1, 2; Pl. XLIX. fig. 3; Pl. LI. figs. 6-8; Pl. LII. fig. 2; Pl. LIN. fig. 6). 
In Pentacrinus asteria (Pl. XII. figs. 4, 8) and Pentacrinus miilleri (Pl. XV. fig. 4), 
the sockets are usually more or less transversely oval in shape and well defined below, so 
as to be almost or entirely limited to the nodal joint. In Pentacrinus decorus, however, 
the articular surface occupies the broad end of a pear-shaped depression, which is con- 
tinued down on to the infra-nodal joint (Pl. XXXVI), and thus gives it a distinctly 
stellate outline even when seen from beneath (Pl. XXXVII. figs. 10, 20); while the 
ordinary internodal joint has a rounded or pentagonal outline. The lowest internodal or 
“ supra-nodal” joint of this species is in no way different from the other internodal joints 
above it; while the infra-nodal is hollowed laterally by the downward extensions of the 
cirrus-sockets(Pl. XXXVI. ; Pl. XXXVII. figs. 8,19). In Pentacrinus wyville-thomsoni, 
however, the lower edge of the socket projects outwards beyond the level of the infra- 
nodal (Pl, XIX. figs. 3, 4). This joint is but little different from those below it, except 
just at its upper edge where it meets the enlarged surface of the nodal joint that rests on 
it (Pl. XXIL. figs. 21, 22). The supra-nodal joint, on the other hand (Pl. XXII. fig. 17), 
is not so round as the remaining stem-joints (fig. 23); for it is slightly hollowed by the 
upper portions of the cirrus-sockets, and thus more nearly approaches the shape of the 
upper face of the nodal joint on which it rests (fig. 18). 
This condition becomes still more marked in Metacrinus, which has relatively low nodal 
joints with wide and comparatively inconspicuous sockets (Pl. XXXIX. fig. 3; Pl. XLI. 
figs. 1, 5, 15; Pl. XLVII. figs. 1, 2; Pl. LI. figs.6-8; PL LID. fig. 2; Pl. LIL fig. 6); 
just as in Pentacrinus asteria and Pentacrinus miilleri (Pl. XIII. figs. 4,8; Pl. XV. 
fic. 4); but both supra- and infra-nodal joints share in the formation of the cirrus-socket, 
“oe 
