l'g22. No. 5. ARIONIDAE OF NORWAY. 



The first clear description — with a figure — was produced by Drapar- 

 NAUD (1805I and the name given to it by that author {siiô/iisciis — brownish) 

 is now common!}' used. 



Description. External characters (fig. i and PI. I, 2 — 3I. 



With regard to the size, Avion siihfiisciis is intermediate to Arioii 

 atcr and the following smaller species, the usual length of adult, fully 

 extended specimens being up to 6 cms., only rarely does it attain a length 

 of 8 cms. The body is more slender than that of Arioti atcr. When 

 irritated the larger specimens may assume the semi-globular shape which 

 often is considered onlv to characterize Arion atcr. The foot-fringe has 

 the same system of dark transverse furrows as the latter species. 



The colour varies greatly. As a rule it is darker on the back, with 

 paler sides, rarelv the reverse. Inside each oS. the brownish longitudinal 

 bands is a paler stripe, and the younger the animal, the broader are these 



Fig. 13 — 14. Jaws of Arion sub/usciis, ^-1. 



pale Stripes at the expense of the darker mid-dorsal part. Often the colour 

 is unequal!}' concentrated, so as to constitute indistinct and irregular spots. 

 Very commonly the total impression of the colour is a dirty yellowish-brown, 

 at other times varying from yellow-grey to dark brown and even almost 

 black. The orange mucus greatly influences the colour, and a pale brown 

 specimen may turn grey when the mucus is removed. 



The tentacles are dark, but paler than the bluish-black colour which 

 distinguishes the voung ones of Arioii atcr. — The foot-sole is of an almost 

 uniform pale yellowish colour. 



As the animal grows older, the pigmentation concentrates under the 

 respiratory orifice, and, encircling the latter, forms a lower side-branch to 

 the right shield band ; accordingly this appears swollen around the orifice, 

 in reality in such a way that the pigmentation inferior to the respiratory 

 orifice is fainter than the superior, its distinctness being exaggerated in 

 the diagram I fig. i). 



The jaw (fig. 13 — 15) is semi-lunar. Often, but not always, the 

 median part of the concave edge is convex. It is as much as 2 mill, broad, 

 of a brown colour, paler in young darker in older specimens, and is there 

 darkest in the lower part along the cutting edge. The lines parallel to 

 this varv in situation and in distinctness. 



