24 KUIDTHJOF ØKLANI). M.-N. Kl. 



man}' cases tlicsc pair liiilc yoimj^ ones may be distinguished from other 

 species by the dark bluish tentacles. Later on, when the animal has attained 

 a length of 3 4 cms., the back becomes pigmented, and at last the sides 

 of" the body and of the foot-sole blacken. The pigment first appears 

 between the tubercles of the skin, and slowly intensifies. Very soon the 

 foot-fringe acquires its peculiar aspect, owing to the alternately pigmented 

 furrows. Animals about 5 cms. — according to Kunkel equal to about 

 four months — generally have a more or less dark slate-blue colour. 



Howewer several exceptions to this development occur. Not rarely the 

 little young ones assume a greenish tint. During growth dark longitudinal 

 bands may appear, especially on the shield. These are described under 

 the external characters of the species. 



At an age of 10 — 11 months, the increase in length is brought to a 

 conclusion, but the weight still increases (KCnkel). The animals attain an 

 age of litde more than a year, and it appears that only the young ones 

 survi\-e through the winter. 



Arion subfuscus (Draparnaud). 



?(i774 Liiiiax /lisais Müller, Verm. Hist., vol. 2, p. 1 1 1. 



1805 „ subfuscus Draparnaud, Hist. Moll., p. 125, pi. IX, fig. 8. 



1822 „ fasciatus, var. e, JT, t], Nilsson, Hist. Moll. Sveciae, p. 4. 



1822 „ ßaviis Nilsson, op. cit., p. 5. 



1853 „ subfuscus Drap. var. ß apud Friele, Norske Land- og Ferskv.-Moll., p. 3. 



1870 Prolepis fusens Malm, Göteborg Vet. Vitt. Samh.'s Handlingar, vol. 10, p. 43, pi. II, 



fig- 3 — 4- 



187 1 ArioH riifus L. apud Westerlund, Expose critique Moll. Suède et Norv., Nova 



Acta Soc. Upsal., p. 32. 



1897 Arion citrinus Westerlund, Synopsis Moll. Extramar. Scandin., p. 41. 



Diagnosis. External characters: usually 3 — 6 cms. long, rarely 8 cms. Colour varying, 

 from dirty grey through yellow and reddish-yellow to dark brown and almost black; generally 

 with paler sides, and small specimens especially often with a pale stripe inside the dark 

 longitudinal bands; foot-sole faint yellowish-white. Mucus orange, its varying profusion 

 causing change of colour in the same animal. No pale mid-dorsal line. Usually a dark 

 brown longitudinal band on each side of the back, the right one enclosing the respiratory 

 aperture ; at any rate this is a good characteristic of larger Norwegian specimens. 



Jaw : in most cases distinguishable by the broad, closely set ribs. 



Radula : the mesocone of the median tooth constricted at the base, and the distance 

 from here to the apex of the mesocone so great as or slightly shorter than the distance to 

 the anterior edge of the basal plate. 



Reproductive organs : atrium (in adults) short, almost iso-diametric. Oviduct long, 

 dilates downwards, either gradually, or suddenly at the insertion of the genital retractor, the 

 latter being situated on the upper half of the oviduct. 



Historical systematics. For a long time this species was mistaken 

 for Avion circiiiiiscriptits Johnston and Ariou /lorfcnsis Fér., and it is 

 quite impossible to clear up the complication of synonyms they have caused. 

 An impression of this confusion may be gained when reading Wester- 

 lund's work: Fauna Molluscorum terrestrium et fluviatilium Sveciae, Nor- 

 vegiae et Daniae, 1873. 



