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Show 984 MR. F. A. BATHER ON UINTACRINUS. [Dec. 17, Schlueter (4) says (p. 58), with reference to the corresponding structures in U. westfalicus-" If one were possibly inclined to regard as side-arms, or indeed as pinnules, those rows of plates that ... lie between the arms and the arm-branches, this would be forbidden by the constitution of the plates, since they possess neither a central canal nor a ventral groove, and are united to one another by simple sutures." This argument, though adopted by Neumayr (6), is one I am unable to accept. The absence of a central or axial canal is nothing, since there is none in the arms of many Palaeozoic crinoids: it is, however, a statement that I have been unable to verify in U. socialis so far as the free distal ends of the fixed pinnules are concerned. It is abundantly clear that the free ossicles in the pinnules are of the same character, and are united in the same way, as the ossicles of the subsequent free pinnules. It has also been shown that the fixation of the pinnules is a gradual process. There is therefore no reason to suppose that the fixed pinnules are anything else than pinnules whose bases have become partially fixed, an occurrence by no means rare in other genera. The supplementary plates are of three kinds : interbrachials, interdistichals, and interpiunulars. They are all thin flat plates, and vary considerably in shape and even in number. The interbrachials vary in number from 7 (fide B. H . Hill, 9) or 8 (fide Clark, 8) to 12, e. g. p (fig. 7). In the specimens examined by me, 10 appears to be the most usual number, e.g. y, e, e (tigs. 4, 5), and 1 have never seen fewer than 9. In each interradius these plates all lie above the two adjacent radials, between the fixed primibrachs and opposing fixed secundibrachs 1 and 2, and below the 1st and 2nd ossicles of tbe proximal, outer or interradiad, fixed pinnules. Clark says : " The arrangement of the plates does not vary; seven in an oval band enclose the 8th, or 8th and 9th, according to the number of interradials." This may be accepted as the simplest type of arrangement; but there is considerably more variation than admitted by Clark. The only stable plate is the proximal one, which rests on the upper lateral margin of two adjacent radials, and abuts laterally on tbe two IBrr Its upper margin supports the two succeeding interbrachials, but may also support the central interbrachial between them. To describe the shapes and positions of all the other interbrachials would, considering their variation, be waste of time. It is only necessary to point out that, in the large majority of the specimens before me, e. g. y, e, el, p (figs. 4, 5, 10, 7), there is a single plate lying between the two proximal pinnules and the two subjacent interbrachials, and separated by those two interbrachials from the central interbrachial. This distal plate is not shown by Meek (3), or Clark, or Hill; but it can be seen in Grinneil's (2) fig. 1, although there it rests on a single interbrachial, and not on two as is usual. The particular arrangement of interbrachials figured by Meek and by Clark is unrepresented among the specimens in the British Museum ; but there is no reason to doubt the |
