1894.] ECKDSTODIEMS 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 ealycinal system l 

 (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 Streptophiura? 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 ealycinal plates, the existence of 

 which in true Cladophiurans has only indistinctly been hinted at 

 by Mr. Lyman : but the result is that we have an almost typical 

 Zygophiuran disc, above. On the lower surface the arrangement 

 of the mouth-plates (Plate XXVII. figs. 4 & 5) is most nearly 

 paralleled among known forms by Trichaster pahmfems, 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 new family. But, if it be true that 

 " coelum, non animum, mutant qui trans mare currant," it is 

 equally true that the " f ugaces 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 Cladophiurse is 



1 That is, by those who accept the doctrines first broached by Loven and 

 enforced with such vigour in this country by nay lamented friend Dr. Herbert 

 Carpenter. I understand that there is, among the younger workers, some 

 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 Aster ina 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 i* obvious enough 

 that Johannes Muller, the first user of both the terms, had no idea of any homo- 

 logies between the Crinoid calyx and the Ophiuroid disc. 



3 Ib it quite certain that whal are called radial shields in Cladophiurans arc 

 homologous with the parts called by the same name in Zvgophiurans? 



1 P. Z. S. 1892, p. 180. 



