™ 
REPORT ON THE CRINOIDEA. $1 
mentioned figure ; and it is too small to show the very numerous water-pores piercing 
the ‘plates, some of which have as many as twelve or fifteen openings. They are much 
less abundant in Metacrinus cingulatus and Metacrinus angulatus (Pl. XX XIX. fig. 2), 
while the plates are also smaller. 
The disk of Metacrinus differs from that of Pentacrinus in the greater irregularity of 
its ambulacra; the branches of which proceeding to the large lower pimnules often come 
off directly from the primary groove-trunks, or even from the peristome itself. This is 
especially well shown in Metacrinus angulatus and Metacrinus nobilis (Pl. XXXIX. 
figs: 2): Plo XM tie: 43)- 
Another point of difference is the relatively larger size of the anal tube in Metacrinus, 
which is well shown in Metacrinus nodosus (Pl. L. fig. 2). It may occupy the whole of 
the interpalmar area in which it lies, and is often considerably inflated, so as to be a 
somewhat prominent object on the surface of the disk. It is erroneously represented as 
perfectly bare in the figure of Metacrinus nodosus, and this actually seems to be the case at 
first sight. Closer examination shows, however, that its apparent bareness is really due 
to the smoothness and very close approximation of the plates which cover it."" They are 
thinner than the corresponding plates in the other interpalmar areas, and form a smooth 
continuous pavement over the whole of the lower part of the tube, becoming more 
nodular and irregular towards the top. The whole appearance of the anal tube in this 
species forcibly recalls Buckland’s well known figure of the “abdominal integument” of 
Extracrinus briareus.? There are indications of this close pavement on the anal tube of 
Metacrinus nobilis (Pl. XLUL. fig. 3); and it is better shown im a curious specimen of 
Metacrinus angulatus (Pl. XXXIX. fig. 2), which has a smaller supplementary anal tube 
by the side of the larger one. 
The plates of the pinnule-ambulacra in Metacrinus are better differentiated on the 
whole than those of Pentacrinus. For in the outer parts of the pinnules, at any rate, the 
covering plates rest upon a row of distinct side plates (PI. XLVIL fig. 11; PL LL 
fies, 11, 12; Pl. LIL. figs. 5, 6), and not upon an almost undivided band of limestone as 
in most species of Pentacrinus (Pl. XIII. fig. 15; Pl. XXXVIL fig. 23). 
In the lower parts of the rays and arms the anambulacral plating of the disk extends 
outwards at the sides of the ambulacra, in which the arrangement of plates is confused and 
indefinite (Pl. XLI. fig. 13). Farther out, however, where the zig-zag course of the 
ambulacrum (still distinctly above the arm-groove) is more marked, and the ambulacral 
plates less abundant, the elongated shape of the plates immediately bordering the groove 
is more distinctly visible (PI. XLI. fig. 4). In most species their extremities gradually 
become bifid, as is well shown in Metacrinus angulatus and Metacrinus murray: 
(Pl. XXXIX. fig. 13; Pl. XLI. fig. 14). Both of these, especially the former, have the 
1 See the remarks on the disk of Pentacrinus wyville-thomsont, ante, p. 76. 
2 Geology and Mineralogy, vol. i. p. 439 ; vol. ii. pl. 51. fig. 2. 
(ZOOL. CHALL. EXP.—PART XXxi1.—1884.) li ll 
