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Show 442 MR. W. E. COLLINGE ON EUROPEAN SLUGS. [May 4, mens showed a peculiar series of constrictions in the sperin-duct, and slight constrictions in the free-oviduct. 3. Seven specimens from Northern Italy (precise locality uncertain). The difference from the typical form was very slight, the most marked character being the gradual tapering of the free-oviduct (Pl. X X X . fig. 9). 4. Nine specimens from St. Andrews, N.B. (Pl. X X X . fig. 10). In one example the sperm-duct had the peculiar form figured, and was sharply marked off from the vas deferens. 5. Twenty-seven specimens from Ireland (Pl. X X X . fig. 11). In two examples in this collection tbe vas deferens was sharply marked off from the sperm-duct, which approached in form that mentioned in the previous collection (cf. fig. 10). 6. Thirteen specimens collected at Bern, Switzerland (Pl. X X X . fig. 12). Three specimens showed the globose form of the lower portion of the sperm-duct, and differed somewhat from the type in the form of the free-oviduct. It will thus be seen that out of 1223 specimens of A. subfuscus, Drap., and A. hortensis, Fer., from 235 localities, in only 26 specimens were variations discovered from 10 localities, and three of these were presumably not constant, being found only in individual specimens. I a m of opinion, therefore, from these and other investigations (9) that the form and position of the generative organs in these two species are of the greatest value, and exceedingly reliable for purposes of specific distinction. Further, wherever the form and position of these organs differed, and these differences were constant in a given number of specimens, I should unhesitatingly describe them as distinct species, irrespective of external colouring or markings. Of course, to anyone acquainted only with the form of the generative organs in a very few species, or a small number of any given species, slight differences are apt to receive a wrong interpretation, and an undue importance is very often attached to such; but the experienced malacologist knows that slight differences due to age, season of the year, &c. are constantly found, and are just as worthless as permanent and well-marked differences are valuable. Until, therefore, the variations in any particular species or group of species are shown to be very great, I do not think we can do better than retain the form and position of the generative organs as our basis for specific distinction. 3. A Reversion of a Colour Variation. A few years ago M r . Gain described some very interesting observations upon the colour changes Arion Intermedius, Norraand, undergoes (12), and later I have noted similar colour variations in other species. The present case is, I think, still more interesting than either of the above. In the autumn of 1896 I received a specimen of the white |