OCR Text |
Show 1895.] MR. F. A. BATHER ON UINTACRINUS. 989 The joints between the brachials are of two kinds : perforate articulation, and syzygial suture. The perforate articulation is represented in figs. 2, 3, 6, 7, 8, 9, 10 of PL LIV. The axial canal perforates the brachial at a very short distance from the bottom of the ventral groove. Two parallel ridges run across the under or proximal surface of the brachial, coalescing around the axial canal. These ridges fit into a corresponding groove on the upper surface of the subjacent brachial, which groove itself appears to be bounded by slight ridges. N o specimen shows a single fulcral ridge, with median perforation, such as is shown in Clark's (8) pi. ii. fig. 1 e. Owing to the state of preservation, and to the difficulty of seeing more than one side of any brachial, one cannot certainly distinguish betw-een the proximal and distal surfaces, except when there is other evidence available. The direction of the ridges is not straight across tbe width of the brachial, but is diagonal; and I think that on the distal surface the end towards the pinnule-bearing side is moved dorsalwards, as is the case in Pentacrinus. Since the pinnules alternate from right to left, it follows that the direction of the diagonals must also alternate; so that, as is actually the case, the successive diagonals must lie almost, or quite, at right angles to one another (compare figs. 2 & 10 of PL LIV.). This skewing of tbe ridges is quite obvious and well-marked over the greater part of the arm ; but in the more proximal region, where the brachials are still wdde, and have rounded sides to the ventral groove, this skewing is not so marked (PL LIV. fig. 8). A most careful search has, however, failed to disclose a single brachial with a symmetry like that of Clark's (8) pi. ii. fig. 1 e, although one can find in the proximal region brachials more like Schlueter's (4) pi. iv. fig. 5, a, b, which is still asymmetrical (see P L LIV. fig. 6). Sometimes half the ridge appears to be more skewed than the other half, as in PL LIV. fig. 8. The position of the muscles and ligaments is not easy to determine. There was, no doubt, a bundle of ligament-fibres on the outer or dorsal side of the ridge; and according as the ridge was twisted to left or right, the area of attachment for these fibres must have moved alternately right and left. There is a faint depression over this part of the joint-surface. One would naturally assume a bundle of muscle-fibres in the area on either side of the ventral groove, and ventrad of the ridge; but when the ridge is skewed this area becomes large on one side, and almost non-existent on the other. In the larger area one can easily make out a depression for the attachment of the fibres; while in the smaller area one can often distinguish a strong elongate groove, parallel with the fulcral ridge, and this groove must represent the concentrated area of attachment of the fibres of this side. It is clear that this skewing of the fulcral ridges must have given the arms great power of motion, not merely up and down as in Antedon, but from side to side. In tbis respect the arrangement is analogous to that in the stem of the Bourgueticrinidae, and, P R O C . Z O O L . Soc-1895, No. L X I H . 63 |