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Show 416 PROF. F. J. BELL ON THE ECHINOMETRIDsE. [Mar. 15, It may, at the present juncture, be convenient to recapitulate and extend the results of recent investigations into the characters of the regular Echinoidea. A classification of the regular Echinoidea is not, as it seems to me, quite so impossible a matter now as it was a few years ago ; the discovery, by Mr. Charles Stewart1, of the internal gills of Cidaris, and the extension and independent confirmation of that result by Dr. Hubert Ludwig2, justifies us in accepting the division into Branchiata and Abranchiata, proposed by the latter naturalist3. Although Johannes Miiller had distinctly denied the presence of external gills in Cidaris4, Prof. Alex. Agassiz discovered gill-cuts in the figures of that illustrious anatomist, but only, I fear, by reflecting on the character of the artist, who represents five slits in the median line of the interradial areas; to this, however, Dr. Ludwig has already directed attention. Readers of the just-mentioned naturalist's essay will remember that he proposes to separate the Echinothuridse from the rest of the branchiate regular Echinoidea on the ground of the difference in the characters of the buccal plates. Unfortunately the British-Museum collection contains no specimen of Asthenosoma, although an American collection is in possession of a specimen "which the Museum owes to the kindness of Prof. Thomson, collected by the Porcupine Expedition ;" and I am therefore unable to give any independent judgment as to the point at issue between Sir W . Thomson and the writer of the just-quoted sentence on the one hand, and Dr. Ludwig on the other. To say nothing of the fact that the Porcupine Expedition was fitted out at the national expense, the present state of the question affords ample evidence of the advantage of rare and typical specimens being deposited in a central and national institution. Conflicting as the statements are, those of Dr. Ludwig are so explicit, and are made with so distinct a knowledge of the opinions of his predecessors5, that I think it is, for the present at any rate, the view to which one ought to incline. The Echinothuridse, then, though Branchiata, are distinguished from the rest by having more than one pair of each series of ambulacral plates carried on to the buccal membrane ; they may consequently be distinguished as a polylepid as compared with a decalepid series. This decalepid series includes the Diadematidse, the Arbaciadae, the Echinidse, and the Echinometridse, together with the Salenidse. These last are at once to be separated off from the rest by the characters of their apical area; they are pala?oproctous forms, as 1 Trans. Linn. Soc. (2), i. p. 569. 2 Zeitsohrift fur wiss. Zool. xxxiv. pp. 70-87. a Prof. Alex. Agassiz gives no information, in his preliminary diagnosis, as to the gills of Aspidodiadema. 4 Abh. Berl. Akad. 1853, p. 146. H e speaks of "ein ganz fundamentaler und bis jetzt nicht beacbteter Gegensatz zu den Cidariden." The possession of buccal plates being a characteristic of the Desmosticha, the differences which obtain with regard to them are to be insisted on in the arrangement of the constituent families. |