442 MR. W. E. COLLINGE ON EUROPEAN SLUGS. [May 4, 



mens showed a peculiar series of constrictions in the sperm-duct, 

 and slight constrictions in the fi-ee-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 (PI. XXX. fig. 9). 



4. Nine specimens from St. Andrews, K.B. (PI. XXX. 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 (PI. XXX. fig. 11). 

 In two examples in this collection the 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 XXX. 

 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. suhfuscus, 

 Drap., and A. Jwrtensis, Per., from 235 localities, in onty 26 

 specimens were variations discovered from 10 localities, and three 

 of these were presumably not constant, being found only in 

 individual specimens. 



I am 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. Purther, 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 inter- 

 pretation, 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 Mr. Gain described some very interesting 

 observations upon the colour changes Arion intermedius, Normand, 

 undergoes (12), and later I have noted similar colour variations 

 in other species. 



The present case is, I think, stiU more interesting than either 

 of the above. 



In the aatumn of 1896 I received a specimen of the white 



