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Show 344 DR. GWYN JEFFREYS ON THE MOLLUSCA OF THE [May 20, original. Upwards of forty years ago the late Edward Forbes proposed to distinguish species in the same way. But it is notorious that the relative value and constancy of these characters have yet to be determined, and that there is no standard of reference by which naturalists can be guided in adjudging some forms to be species and others to be varieties in different genera. Every naturalist, whether of the old or the " new" school, must form his own opinion. I have already, in the first chapter of the introduction to m y work on ' British Conchology ' (pp. 18-19, 23), fully stated m y views on this difficult and hitherto unsettled question. In the present species the apical whorls are pinched up and narrower than the rest of the spire; the mouth in perfect specimens resembles that of Pherusa gulsona. 4. ACLIS VENTROSA, Jeffreys. A. ventrosa (Jeffr.), Friele, Bidrag til Vestlandets Molluskfauna (Vid. Forh. 1875, separate copy), p. 5, t. i. f. 7, 7a, 7b. 'Porcupine' Exp. 1869: St. 23a. 1870: Atl. 16. Distribution. Lofoten I. (6r. 0. Sars), Bergen (Friele) ; 260- 300 fms. More conical and broader at the base, and with the whorls more rapidly increasing, than A. walleri. It belongs to the genus Hemi-aclis of G. O. Sars. PHERUSA GULSON^E, Clark. Chemnitzia gulsona, Clark in Ann. & Mag. Nat. Hist. 3rd ser. vi. p. 459. Aclis gulsona, B. C. iv. p. 106 ; v. p. 210, pi. Ixxii. f. 5. 'Porcupine' Exp. 1870 : Atl. St. 27, 28. Fragments only. Distribution. British and Irish coasts from Shetland to Guernsey, Vigo Bay (McAndrew), Palermo (Monterosato), Madeira (Watson) ; 20-103 fms. Fossil. Pliocene; Coralline Crag, Sutton (S. Wood). See ' British Conchology ' for the description of the animal and shell, which shows the peculiar characters of this mollusk. I had there suggested the generic name Menippe; but as that name had been previously employed in the Crustacea, I venture to substitute Pherusa, which was at one time given to it by Mr. Clark, the discoverer of the shell. I cannot explain the meaning of this name. Family XVII. PYRAMIDELLIDJE. Apex having a sinistrorsal and exposed spire. It is unnecessary, if not a waste of time, to recapitulate the facts and arguments which I used in 'British Conchology' (vol. iv. pp. 108-111) for the purpose of proving that the subdivision of Odostomia into several genera is really unscientific, and is not based on a single valid character. The presence of a "tooth" or columellar fold, the length of the spire, and the kind of sculpture (if any) are so variable, that it is quite impossible to distinguish or separate certain |