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Show 436 MR. F. J. BELL ON ECHINOCIDARIS. [May 6, 5. Note on the Number of Anal Plates in Echinocidaris. By F. J E F F R E Y B E L L , B.A., Magdalen College, Oxford, Zoological Department, British M u s e u m , F.Z.S. [Received April 23, 1879.] If any one anatomical fact was thought to be certain with regard to the Echini, it was the presence, as a constant mark of differentiation, of four plates and four plates only in the anal region of the species of the genus Echinocidaris. Thus not only does the diagnosis ofDesmoulins (1835) include the words " pieces terminales anales au nombre de quatre seulement," and that of Gray (1835, Arbacia) '•'anus valvis quatuor spiniferis tectus," but the definition given by Prof. Alex. Agassiz1 of the family Arbaciadaa states among other characters that " the anal system consists of only four large triangular plates." Prof. Troschel, in an elaborate article on the genus 2, exhibits not the slightest doubt as to tbe validity of this character, and expresses himself in the following words : - " Das Periproct ist durch vier dreieckige Platten geschlossen " 8. In a later publication than bis ' Revision' (in the Zoological Results of the Hassler Expedition 4 ) , Prof. Alex. Agassiz sounds the first inharmonious note. Troschel (so certain was he of the great value of these four anal plates) had, on account of the presence of five anal plates in some Parasalenice, separated them from the Echinocidaridse, in which family, as he imagined, Agassiz had placed them. Roused, apparently, by his criticism, the American naturalist took the opportunity afforded him by the arrival of additional specimens of E. dufresnii to point out "that it is quite remarkable tbat in the few specimens existing in the British Museum and in our collection, there should be two specimens having five anal plates instead of the normal number of four in the other species of the genus"5. As there are two specimens in the Britisb-Museum collection in which there are five anal plates, and as, on the other hand, the collection made by Dr. Cunningham, " of quite a number of specimens," passed, as Prof. Agassiz informs us, through his hands before the • Revision ' was published, and that without the peculiarity in question being there noticed, it would be possible to read the (not too perspicuous) sentence which I have just quoted in any one of the three following ways :- (J) Both the specimens referred to are in the British Museum; (2) neither specimen is in the British Museum ; or (3) there is, to Prof. Agassiz's knowledge, one specimen in each of the collections mentioned. I am inclined to think that this last construction is the one which the words were meant to bear; and I am supported in this belief by the difference between the two specimens from Dr. Cun- 1 Revision of the Echini, p. 399 (1872-1874). 2 Archiv fur Naturgeschichte, xxxviii. p. 293, xxxix. p. SOS. 3 Op. cit. xxxviii. p. 298. 4 Illustrated Catalogue, Mus. Comp. Zool. viii. n 6 (1874) 5 Cat. M. C. Z. viii. i. p. 6. |