1897.] MK. W. E, COLLINGE ON EmOPEAN SLUGS. 443 



variety of A. em^nricorum, Per., from Eoundhay near Leeds. It 

 was of an almost pure white, not the creamy-yellow variety which 

 is much commoner. Tor some months the specimen was kept 

 alive beneath a bell-jar on a large iiower-pot, and liberally supplied 

 with leaves of lettuce, cabbage, &c. During this period I wit- 

 nessed a complete reversion to the typical black form. After I 

 had kept the animal for about five weeks, I noticed that it looked 

 much dirtier than previously, and about the seventh week it was a 

 very dirty white, which quickly changed into a faint sepia. About 

 the ninth week it became very inactive, and for three weeks, when- 

 ever I examined it, it was drawn up in the peculiar arched form so 

 characteristic of this and a few other species of Arion. During 

 this time a lai-ge amount of thick dark yellow slime was exuded 

 from the caudal mucous pore, and remained over the postero-dorsal 

 region of the body. Little by little the colour deepened, and the 

 animal became more active about the middle of the fourth month, 

 by which time it was almost black : by the end of the fifth month 

 it was impossible to distinguish it from the typical black forms. 



I have previously seen examples of this Slug undergo slight 

 changes in colour, such as the black forms becoming much lighter, 

 a dark sepia ; but I know of no instance of so complete a change 

 as the one here described, viz., a complete reversion to the normal 

 colour from a pure white. 



4. The Specific Validihj of Arion fuscus, Mull. 



Through the kindness of Signor Pollonera, I was able in 1892 

 to examine specimens of this species, and from the slight diifei'ences 

 I was able to detect in the form of the generative organs in these 

 specimens, I was inclined to regard it as a variety of A. suhfuscus, 

 Drap. (7). Since then I have been able to make a more careful 

 examination on some better material, for which I am indebted 

 to the kindness of Herr Joseph F. Babor, of Prague University ; 

 and I am of now of opinion that, from the diffei'ences in the form 

 of the generative organs, it must be regarded as a distinct species. 



Abion euscus, Midler, 1774. 



Limax fuscus, Mliller, Verm. Hist., 1774, vol. ii. p. 11. 

 Arion fuscus, Morch, Vidensk. Medd. naturhist. For. Kjoben- 

 havn, 1863, p. 273. 



Prolepis fuscus, Malm, Skand. Land-Sniglar, 1870, p. 43, pi. ii. 



fig- 4- 



Arion rufus (partim), Westerl., Expose critique Moll. Suede et 



Norv., 1871, p. 32. 



Arion citrinus, Westerl., Expose critique Moll. Suede et Norv., 

 1871, p. 35. 



Arion stabilei. Poll., Atti Ace. Sci. Torino, 1885, p. 28. 



Arion fuscus. Poll., Atti Ace. Sci. Torino, 1887. 



This species is much smaller than A. suhfuscus, with the bands 

 on the mantle distinct ; the lines on the foot-fringe vary from dark 



