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Show 1894.] ECHINODERMS OF MACCLESFIELD BANK. 409 When, however, we come to closely examine the disc we find it to present an arrangement of plates that is quite unknown in any Astrophytid; for there is on it a set of plates which cannot be supposed to be anything but the remnants of a calycinal system' (see fig. 3, c & r); the centre of the disc is occupied by a rounded plate, and midway between it and the base of every arm but one there is a plate which cannot but be the homologue of the radial plate ; just as distinctly there is to be seen at the base of the arms a pair of plates which are surely the so-called radial shields 2. Though radial shields are not diagnostic of Ophiuroids, for they are, at any rate, absent from such simple Streptophiurae as Neoplax, they are exceedingly characteristic of the group, and are of large size in Cladophiurans 3. In the specimen before us they exhibit some irregularity, but they do not present the characteristic of the Cladophiuran ; they are not "rippenartig" and they do not extend over the whole semi-diameter of the disc. Their smaller size may be correlated with the presence of calycinal plates, the existence of which in true Cladophiurans has only indistinctly been hinted at by Mr. L y m a n ; but the result is that w e have an almost typical Zygophiuran disc, above. On the lower surface the arrangement of the mouth-plates (Plate X X V I I . figs. 4 & 5) is most nearly paralleled among known forms by Trichasier palmiferus, and I know of nothing resembling it that has been detected in any fossil form; the distinctness of the two halves of the oral apparatus is very marked, and must be supposed to be a primitive character. With regard to the systematic position of this very remarkable form, I feel inclined, after much reflection, to adopt an attitude of reserve ; some years since I should not have hesitated in taking it to be the type of, at least, a n ew family. But, if it be true that " ccelum, non animum, mutant qui trans mare currunt," it is equally true that the " fugaces anni" carry away with them the cause of many a bad new species or group. It is possible still to use the diagnoses propounded in 1892 for the Cladophiurae *, as the size and extent of radial shields is not there used as a diagnostic character. So far as the descent of the Cladophiurae is 1 That is, by those who accept the doctrines first broached by Loven and enforced with such vigour in this country by my lamented friend Dr. Herbert Carpenter. I understand that there is, among the younger workers, BOine scepticism as to the validity of these homologies. [Since this was written Mr. E. W . MacBride has published an abstract of his observations on the organogeny of Asterina gibbosa (Proc. Roy. Soc. Lond. liv. pp. 431-6). I am sure many morphologists await with interest the proofs of his statement that there is no homology between the abactinal poles of Crinoids and Asteroids.] 2 If we are to continue to recognize homologues of the radials of the Crinoid calyx (see P. H. Carpenter, Quart. Journ. Micr. Sci. xxiv. (1884) p. 1), it might be well to make some alteration in nomenclature, as the presence of " radial" plates and " radial" shields on the same disc is confusing. It is obvious enough that Johannes Muller, the first user of both the terms, had no idea of any homologies between the Crinoid calyx and the Ophiuroid disc. _ A Is it quite certain that what are called radial shields in Cladophiurans are homologous with the parts called by the same name in Zygophiurans? 4 P. Z. S. 1892, p. 180. |