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Show 994 MR. F. A. BATHER ON UINTACRINUS. [Dec. 17, epizygals gradually diminish slightly in height, till they become about two-thirds the height of an ordinary brachial, rarely less (PI. LIV. fig. 11). It is often very hard to decide whether two adjacent ossicles are united by syzygy or no, so little do they differ in outward appearance from the neighbouring brachials. Occasionally the syzygial joint is a little less curved, as though there were some attempt at dovetailing, a suggestion of the structure in Pentacrinus naresianus and P. blakei. The pinnules.-The law of pinnulation for the fixed pinnules has been discussed fully. That for the free brachials is the same as obtains in most modern crinoids : each ossicle, except the hypozygals, bears a pinnule, and the pinnules are on the right and. left sides alternately. Viewed from the side of the arm, as in fig. 11 of P L LIV., the proximal ossicle of each pinnule appears sub-crescentic in outline, and lies in a corresponding semicircular notch, excavated in the upper corner of its brachial. The lower corner of the succeeding brachial is also slightly excavated, and the pinnulars fit closely up against it. In the case of a syzygy, the notch for the proximal pinnular may extend a little below the epizygal and include a small portion of the hypozygal. W h en seen from the back of the pinnule, each pinnular, with the exception of the proximal one, has its upper and lower margin parallel to each other ; but seen from the side each ossicle has a sub-triangular outline, the base of the triangle being dorsal. This is seen to a slight extent in fig. 13 of PI. LIV. In other words, each pinnular decreases in thickness towards the ventral side of the pinnule; and this enables the pinnule to curl upwards over the ventral groove of the arm. This feature is more marked in the proximal region of the pinnule than in the distal. As is usually the case with both arms and pinnules in the Crinoidea, the proximal ossicles of each pinnule are wider than high, while the distal ones are higher than wide. There are at least 30 ossicles in each pinnule ; in some, 40 can be counted. In some cases the pinnulars show a concavity of the sides like that in the brachials (PL LIV. fig. 13). O n the ventral side each pinnule has a shallow groove (PL LIV. fig. 14). This was probably roofed by covering-plates, though none are preserved in the fossil. On either side of the groove, and at right angles to it, is a slight depression or channel in each pinnular. These may have been for the passage of the tentacles, or they may represent articular depressions for tbe covering-plates such as 1 have figured for Cyathocrinus acinotubus and C. ramosus1. The objection to the latter interpretation is that the covering-plates must in that case have been rather too large and solid to have so entirely disappeared. The articular surfaces of the pinnulars (PL LIV. fig. 12) appear to show an axial canal separate from the ventral groove, and on either side thereof a depression for attachment of ligament, i. e. a 1 " Orinoidea of Gotland," i, figs. 208 & 255, in Svensk. Vet.-Atad. Handl. xxv. 2 (1893). |