32 



FKIDlllIOl- ØKLANI>. M.-N. Kl. 



Arion circumscriptus Johnston. 



?(r822 l.iiiDix fascialns, var. y Nilsson, Hist. Moll. Svcciae, p. 4I. 



1828 ylnoii circiiiiiscri/>/tts Johnston, Edinburgh New Philos. Journ., vol. 5, p. 76. 



1852 „ /riicn/i/inciis NoKMANi), Descr. Six Limac. Nouv., p. 6. 



1853 Liiiiax suhfitsais Drap. var. a apud Friele, Norske Land- og Ferskv.-Moll, p. 3. 

 1868 Arion hourguignati Mabille, Rev. et Mag. Zool., vol. 20, p. 138. 



1870 Prolrpis lioitensis Malm, Guteborg Vct. Vitt. Samh.'s Handlingar, vol. 10, p. 49, 



l'l. II, fig. 5- 



1871 Arion hortnisis Férussac apud Westerlu.vd, Expose critique, Nova Acta Soc. 



Up.sal., p. 35. 



1872 — „ — — Jen-,en, Indberetning. 



1886 — „ — — EsMARK, Journ. of Conch, vol. 5, p. 103. 



Diagnosis. External characters : rarely more than 4 cms. The colour usually either 

 yellowish grey with paler sides or of a uniform bluish grey ; foot-sole pure white. Along 

 tho middle of the back a single row of pale tubercles, especially conspicuous in the young. 

 Mucus never deep yellow. The right shield-band entirely on the upper side of the respira- 

 tor'/ orifice, and in case of a pigmentation below the band, no distinct lateral branch en- 

 closing the orifice is formed. 



Jaw : often quite like the jaw in Arion hortensis, with about a dozen slender ribs. 



Radula : has a great similarity to that of Arion snbfusciis and Arion hortensis, but 

 often differs, by respectively the mesocone of the median tooth being slender, with more 

 distinct secondary cusps, and the marginal teeth as a rule having two cusps. 



Reproductive organs : greatly varying, but with a characteristicly long ribbon-shaped 

 atrium, and the bladder of the receptaculum generally having a pointed end. 



Historical systematics. This species, too, has caused much 

 trouble owing partly to the older inadequate descriptions, partly to diffi- 

 culties in defining it in relation to its allied species. In Scandinavia and in 

 the British Isles there is used the name of dr. G. Johnston [circitniscriptiis 

 — enclosed, i. e. by the lateral bands), otherwise the more recent name oj 

 Mabille [Boiirgiiigtiati after Bourguignat) is the most common. 



In Norway it was called by Westerlund Arion hortensis, and Clessin 

 (1884) was the first to designate it under its right name. Later on Bir- 

 GiTHE EsMARK and O. S. Jensen collected it in many localities, but, following 

 the example of Westerlund, published it under the name of Arion hor- 

 tcnsis, a species from middle Europe which only very recently has proved 

 to belong to the Norwegian fauna, occurring only in scattered localities. 

 I have verified the erroneous determinations in this case by examining the 

 collections of Esmark and Jensen (in the zoological Museum at Toien, 

 Kristiania). 



Description. External characters (fig. i and PI. I, 4 — 6). 



The actual size may be useful in avoiding mistakes as regards the 

 species mentioned above, Arion circ/iniscripiiis being considerably smaller 

 and only rarely exceeding a length of 3 — 4 cms. fully extended; out of 

 about 800 living specimens I found only three that were about 5 cms. 

 Especially in the common variety the body is proportionally high and 



