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Show 398 DR. G W Y N JEFFREYS ON MOLLUSCA OF THE [Apr. 16, tula, as Mr. Gr. B. Sowerby, Junr., and Mr. Reeve have done. As to Terebratulina and Waldheimia, Mr. Davidson some years ago considered both subgenera of Terebratula ; but he is now of opinion that Waldheimia is a distinct genus, because (to use his own words) " the recent or living species are but a handful, in comparison with the fossil species, and these last must be taken into account when making divisions of the class, and it would be a very great advantage to retain such divisional groups, which have so many distinguishing characters, and are so numerous." I fully admit the force of his reasoning on the first mentioned ground, and that all fossil species must be included with recent species in the same scheme of classification ; but the latter ground, which depends on the validity of distinctive characters and the relative number of species, is not, in m y opinion, equally tenable. It is notorious that Terebratulina and Waldheimia gradually pass one into another, as well as into the main or typical genus Terebratula ; and 1 contend, with great deference to Mr. Davidson's more experienced judgment, that it would be more advantageous to science not to multiply unnecessarily the generic names, when fewer would answer every purpose. It cannot be pretended that such names have any value beyond being mere symbols or signs to distinguish and recognize certain groups; they are, and must be, quite artificial and arbitrary. The question of number of species in a genus does not appear to me of much importance. Take for instance the genera Unio, Helix, and Bulimus, each of which, even in a restricted sense, contains several hundred species. It may, of course, be possible to divide these and other well-stocked genera into more genera, each containing an equal number of species; and the same method might be adopted with respect to the species, and even with respect to the individuals of every species. But that would hardly be a scientific proceeding; and it might cause much confusion or needless trouble. All genera ought to be equivalent as regards distinctive characters, not as regards the number of species. I will now venture to suggest an arrangement of the few Brachiopoda which are at present known to inhabit the European seas, and which comprise the species procured in the 'Lightning' and 'Porcupine ' Expeditions. The following Table gives not only the names of the species and principal varieties, but also the range of depth and the places where they have been found fossil in the Tertiary formation. |
