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Show 980 MR. F. A. BATHER ON UINTACRINUS. [Dec. 17, the dorsal cup consist of (i.) primibrachs, IBr ; (ii.) secundibrachs or distichals, IlBr ; (iii.) fixed pinnules. By " fixed " one means that the ossicles in each longitudinal series are attached not merely to one another by their upper and lower margins, but also to the ossicles of adjacent series by their lateral margins, thus forming the cup. The precise limits of fixation are not very easy to determine in the fossil state : for instance, the distal extremities of the earlier pinnules were undoubtedly free, although their proximal portions were firmly united laterally to the distichals or to other pinnules ; but tbe exact point at which lateral union ceased cannot be determined, since, even above the limits of strict lateral sutures, the pinnules may have been laterally united by a membrane. The same uncertainty prevents us from saying at what level the arms became free ; but we may say somewhere about the 8th and 9th distichal, IIBr9. The primibrachs (" radials of the first order" in part, or " costals," of some writers) are two in number (see figs. 4 and 5). IBrx is hexagonal, resting by its lower edge on the feebly concave upper margin of the radial, supporting IBr2 on its upper margin, and abutting laterally on two interbrachials on either side. It is slightly less wide than the radial, but about the same height. IBr2 is axillary (IAx) and pentagonal, having on rare occasions an irregularity in the lateral margin, owing to the abutment thereon of more than one interbrachial. The fixed secundibrachs (or fixed distichals) may, as aforesaid, be reckoned at about 8. They are fixed by means of interbrachials, fixed pinnules, and interdistichals. Owing to the origin from some of them of the fixed pinnules, they have a slightly irregular, zigzag arrangement, and those that bear pinnules have somewhat the aspect of axillaries. The law of their pinnulation, in both right and left branches of each arm, may be stated thus: IIBrt, none ; IIBr2, outer; IIBr3, none; IIBr4, inner ; IIBr5, outer ; IIBr6, none ; IIBr7, inner ; IIBr8, outer; IIBr9, none. This, at least, is the arrangement I have found in eight specimens out of ten, e.g. specimens y, d, e,f, g, I (figs. 4, 5, 10). One of the ten specimens, viz. b (fig. 6), varies thus : IIBre, inner: IIBr7, none; IIBr„, outer; IIBr9, inner. The remaining one, viz. p (fig. 7), varies thus : IIBr6, none ; IIBr6, outer ; IIBr., inner ; IIBrg, none ; IIBr9, outer. In both these cases the regular alternation of pinnules remains. It appears that all tbe rays of any individual vary, if they vary, according to the same plan. The above results were gained from an examination of actual specimens in the British Museum : it is instructive to compare them with the published figures. GrinnelPs (2) figures 1 and 2 (my fig. 1) agree with the law so far as the portions in question are preserved. Both of Meek's (3) figures (my fig. 2) agree with it in every particular. Prof. Clark's (8) " Diagram showing the structural arrangement of the plates in the test" agrees with it. Clark's pi. i. fig. la (PL LVI.) shows the following arrangement in two branches :-IIBrs, outer ; IIBr6, none; IIBr7, outer. Not |