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Show 1895.] MR. F. A. BATHER ON UINTACRINUS. 99i In the present paper, at any rate, each brachial is treated as a unit, and the term syzygy is confined to its original meaning, viz., an immovable sutural union between two ossicles. In the fixed region of the arm w e may assume that the distribution of the syzygies is governed by the law of pinnulation above stated, that is to say, the non-pinnulate secundibrachs are taken to be hypozygals. For U. westfalicus this assumption is definitely confirmed by the observations of Schlueter (4), which show that the ossicles in question have a radiate upper joint-surface. In U. sodalis this has not been seen, but in external appearance tbe supposed syzygial sutures are closer and finer than the rest. For IIBr the distribution of the syzygies has been worked out in 16 arms belonging to 5 individuals, and the results are given in the accompanying diagrams (pages 992 & 993). It appears from these that the syzygial sutures occur at successive average intervals of 3, 4, 4, 5, 7, 5, 7, and 8 sutures. The persistent syzygial nature of the 26th suture is remarkable, and is one of those facts that would be obscured by the usual method of counting. The regularity of the syzygies seems to decrease in the more distal regions ; that is to say, although in a single arm the intervals are fairly constant, yet there is considerable difference between individuals, possibly between the several arms of one individual. There is some reason to suppose that the interval between the syzygies is usually less in the proximal portion of the arm than it is in the median portion, but the evidence is insufficient. A long arm-fragment of medium width has syzygies at intervals of 10-15 sutures ; but another much narrower arm-fragment of over 130 brachials has syzygies at intervals of 5-8 sutures, the usual intervals being 6 and 7. The large number of the syzygies and their regular distribution throughout the whole length of the arms, doubtless bear some relation to the animal's mode of life. It is natural to suppose that syzygies have been developed to afford points of easy fracture, such as could never be presented by the muscidar and ligamentary articulations. Thus, when an arm is entangled or is caught by some enemy, the crinoid merely breaks it off and swims or crawls away, happy in the knowledge that a new arm will soon grow from the stump. This explanation has been confirmed by the experiments of J. Walther on Antedon rosacea.1. A Uintacrinus, with its sociable disposition, must often have found its long flexible arms inextricably interlaced with those of its fellows. Only by the syzygies can it have escaped from the too close embrace. In the fixed portion of the arm the hypozygals and epizygals do not materially differ in height or appearance from the other fixed brachials. It would be preposterous to count IIBr3&4, IIBr6&7> or even IIBr , as single morphological units, as would be done *) CC IU on the usual system of counting. After this, the hypozygals and i " Untersuchungen ueber den Bau der Orinoiden," Palaeontographica, xxxii. p. 180. (Oct. 1886.) ^ |
