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



modificatiou of the lower arm-joints above the pinnule-sockets, which was noticed 

 in Metacrinus costatiis, is also visible in Metacrinus nodosus, though to a less 

 extent. 



There is a good deal of difference, however, between the stems of the two species. 

 While the normal number of internodal joints in Metacrinus costatus is seven or eight, 

 that of Metacrinus nodosus is eight or nine, and they are very regularly marked by a 

 faint tubercle in the middle of each side, which is flat and scarcely hollowed at all (PL LI. 

 fig. 8). In Metacrinus costatus, on the other hand, the sides of this stem are almost 

 smooth, or only marked by a few occasional horizontal ridges, while they are distinctly 

 hollowed between the prominent angles (PL XLIX. figs. 3, 4), so that the stem appears 

 to be traversed along its whole length by five rather sharp interradial ridges. In the 

 uppermost and growing part of the stem these ridges are much more prominent on the 

 closely set nodal joints than on the thin internodals which separate them. But as the 

 latter increase in thickness their interradial angles are also enlarged, so that those on the 

 nodal joints are not specially prominent. This character is also visible in Metacrinus 

 nodosus, both in the stem-joints of young individuals (PL LI. figs. 6, 7) and in the 

 growing part of the stem of the more mature specimen. But there are no strong ridges 

 developed at the angles of the internodes, as is the case in Metacrinus costatus, so that 

 those of the nodal joints are always more or less prominent (PL L. fig. 4 ; PL LI. fig. 8). 



The side plates on the pinnule-ambulacra of Metacrinus nodosus are relatively large 

 and pointed (PL LI. figs. 11, 12). They are developed in the same style as those of 

 Metacrinus costatus (PL XL VII. fig. 13) from the somewhat irregular plates of the 

 brachial ambulacra, which are not so bifid as in Metacrinus angidatus (PL XXXIX. 

 fig. 13) or in Metacrinus varians (PL XL VII. figs. 11, 12). 



The young individual of Metacrinus nodosus, which is represented in PL LI. fig. 1, 

 has a slightly tapering stem containing sixteen nodes, at the eighth of which one cutus- 

 socket is undeveloped. The characters of the young stem-joints, which are shown in 

 figs. 2-7, have been noticed already (ante, p. 291). 



Of the four rays remaining in this specimen only one is normal, i.e., composed of six 

 joints, of which the second and fourth are syzygies. In one case the fifth joint is the 

 axillary, and in another the fourth, which is at the same time a syzygial joint like the 

 second ; whde in another ray the fourth radial is not a syzygy, though the fifth is 

 axdlary and united to it somewhat closely, so as to give almost the ajipearance of a 

 syzygy.^ But the presence of a pinnule on the fourth joint shows conclusively that it 

 cannot be the h}^3ozygal of a syzygy, as its homologue is in the next ray. 



This specimen is so young that the palmar axUlaries are with difficulty distinguished 

 from the ordinary joints of the secondary arms ; and in some cases at any rate the}' 

 seem to have been farther from the distichal axUlaries than is usual in the larger indi- 



' A little too mucli has been made of this resemblance to a syzygy in the right hand side of the figure. 



