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Show 990 MR. F. A. BATHER ON UINTACRINUS. [Dec. 17, in a less degree, to the twisted stem of the Platycrinidae. The advantage which such an arrangement of arm-joints confers on a free-swimming crinoid is obvious, since the animal is thereby enabled to progress more rapidly in any desired direction. The origin of this skewing may be connected with the pinnulation of the arm. Each pinnule-bearing brachial is essentially an axillary. A n axillary normally has two joint-surfaces and two fulcral ridges at its distal end ; and these ridges are not parallel to the transverse axis of the ossicle, but converge dorsalwards. As one branch diminishes and becomes a pinnule, the joint-surface on that side also diminishes, while the other joint-surface comes to occupy the greater width of the ossicle, and its ridge becomes parallel with the transverse axis of the ossicle. W e may suppose that in Uintacrinus the slanting of the ridge was maintained, though the ossicle underwent the usual changes. It is of course the case that in the pinnulate arms of other crinoids, e. g. Pentacrinus and Metacrinus, there is an asymmetry of the joint-surface, due to pinnulation, as was long ago well described by Johannes MiiUerx; but I can find no instance of a skewing so marked as in Uintacrinus. The syzygies are of the type common in the Antedonidee (PL LIV. figs. 4, 5). From the periphery of the brachial clearly defined ridges converge to the axial canal. All the ridges do not reach the axial canal, but only alternate ones, or sometimes one in three. The ridges near the medio-dorsal line are the more marked. The space between the ridges seems to be wider than the ridges themselves ; nevertheless I have been unable to distinguish between the upper and lower surfaces of the joint. Sometimes the ridges are slightly channelled. The figure given by Clark (8) seems incorrect in being so symmetrical, and in the meeting of all the ridges around the axial canal. Schlueter's (4) figures of syzygial surfaces in U. westfalicus present a very different appearance, in that they show narrow grooves rather than ridges, which grooves have the same arrangement as the ridges in U. socialis. Both Schlueter's figures represent the upper surfaces of hypozygals, and it might be supposed that the under surfaces of the epizygals were ridged. But, in any case, it is odd that there should be no such grooved appearance in any hypozygals of U. socialis. In describing the distribution of syzygies in an arm, it is the custom to reckon as one ossicle the two ossicles that are united by such a joint, and to transfer the term syzygy from the union to the pair of united ossicles. But " to m y mind ... it is a custom more honoured in tbe breach than the observance," and in another place2 I have given reasons for adopting a method more consistent with both correct terminology and morphological ideas. 1 "Ueber den Bau des Pentacrinus caput medusa," Phys. Abh. Akad. Wiss. Berlin, Jahrg. 1841. See p. 213 and pi. ii. figs. 4 & 13 (1843). 2 " The Term ' Syzygy' in the Description of Orinoids," Zool. Anzeig. vol. xix. pp. 57-61 (Feb. 3, 1896.) |