REPORT ON THE ORINOIDEA. 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 in both cases a variable number of the lower joints 

 (PI. III. 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 joints ; 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 PI. 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 III. 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 PI. I.; 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 feUows 

 of the adjacent arm. 



Other irregularities 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. figs. 14, 15, and PL Vc. 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 (Pis. 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 aU the specimens, but those of the third 



