14 THE VOYAGE OF H.M.S. CHALLENGER. 



and there is uo more direct communication between tlie 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 (PI. XXIV. figs. 8, 9, Id), 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 

 difi'erent species. They are not specially prominent in Pentacrinus asteria (PI. XIII. 

 figs. 4, 8), Pentacrinus miilleri (PI. XV. fig. 4), Pentacrinus hlalcei (PL XXXI. fig. 3), 

 or in Pentacrinus naresianus (PL XXVIII. fig. 2 ; PL XXXa. fig. 6). In Pe7itacrinus 

 decor us, 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 

 ivyville-tliomsoni (PL XIX. figs. 3, 4) the sockets are very jM'ominent, 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 XLVII. 

 figs. 1, 2 ; PL XLIX. fig. 3 ; PL LI. figs. 6-8 ; PL LII. fig. 2 ; PL LIII. fig. 6). 



In Pentacrinus asteria (PL XIII. figs. 4, 8) and Pentacrinus miilleri (PL XV. fig. 4), 

 the sockets are usually more or less transversely oval in shape and weU 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 XXXA^II. figs. 8, 19). In Pentacrinus ivyville-thomsoni, 

 however, the lower edge of the socket proj ects outwards beyond the level of the infra- 

 nodal (PI, XIX. figs. 3, 4). This joint is but little difi'erent 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 XXII. 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 slightl)?^ hollowed hj 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 LIL fig. 2 ; PL LIII. fig. 6), 

 just as in Pentacrinus asteria and Pentacrinus miilleri (PL XIII. figs. 4, 8 ; PL XV. 

 fig. 4) ; but both supra- and infra-nodal joints share in the formation of the cirrus-socket, 



