OL 
REPORT ON THE CRINOIDEA. 207 
ligaments on the dorsal side of the articular ridges would help in the extension of the 
arms again, 
It has already been mentioned that the trivial arms are larger and better developed 
than those of the bivium; but im both cases a variable number of the lower joints 
(Pl. IIL. figs. 6-13) are considerably larger than those which follow them (figs. 14, 15), 
and the passage from one to the other is usually somewhat sudden. On the trivial arms 
there are generally from 8 to 10 of these large, massive jomts; but on the bivium there 
are only about seven, six, or even less. The difference between the two is very well 
shown in the small specimen represented in Pl. IV. The shape of these lower arm-joints 
is rather variable. They may be roughly oblong as is the case with the first two or three, 
or their edges may be more oblique so. as to give them a truncated wedge-like form. 
The more wedge-shaped these joints are owing to the obliquity of their terminal faces, 
the greater is the inequality in the size of the muscle-plates on the two sides of the 
median groove. This inequality is visible in the joints represented in Pl. IIL figs. 10 to 12, 
though it is sometimes still more distinct. The pinnule-socket of such a joint is on the 
thickened upper edge of the higher muscle-plate. The general character of these lower 
arm-joints is much less regular and symmetrical than is the case in other Crinoids, so 
that many of them are more or less of a monstrous nature. In some few cases, indeed, 
the joint is smaller than usual and triangular, not extending completely across the arm, 
so that the joints above and below it come into contact with one another. This is shown 
in various parts of both figures on Pl. L.; and it is comparable to the condition of other 
parts of the same specimen, viz., the way in which the first brachials may partly rest on 
the second radials, or the axillaries on the first radials, as has been already described. 
Sometimes again, a first brachial becomes unusually large, as is shown on two of the 
bivial arms in Pl. I. fig. 1. The inner one of the two bears a small, triangular, second 
brachial, and consequently comes into contact with a similarly large, third brachial along 
its outer edge; but the outer edge of the other second brachial sends a long process 
forward by the side of the next three joints, which are much smaller than their fellows 
of the adjacent arm. 
Other irrecularities of growth appear in the same individual, but they are by no 
means so marked in that shown in Pl. II. This, moreover, shows very well the rather 
sudden diminution in the size of the arm-joints which lose their tubercles and gradually 
become laterally compressed, so that their medio-dorsal edge is tolerably sharp. This 
form of joint is figured in Pl. III. fies. 14, 15, and Pl. Ve. fig. 2. The longest arms 
seem to have about eighteen of them, raising the total number of brachials to between 
twenty-five and thirty. 
The larger, outer sides of all the brachials bear the pinnules (Pls. II., Va., Vb.). 
That of the first brachial is comparatively small, and is attached close to the distal edge 
of the joint; the next pinnule is invisible in all the specimens, but those of the third 
