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 (PI. 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. XLVII. fig. 13) from the somewhat irregular plates of the
brachial ambulacra, which are not so bifid as in Metacrinus angulatus (Pl. XXXIX.
fig. 13) or in Metacrinus varians (P). XLVII. figs. 11, 12).
