1922. No. 5. 



ARIONIDAE OF NORWAY. 



33 



broad; the foot-fringe is rather inconspicuous as it lacks the dark trans- 

 versal lines of both the preceding species. 



The colour in the common variety is of a yellowish grey, with paler 

 sides and with an orange band, more or less sharply defined, just outside 

 the dark lateral bands; in the variety nigra nov. var., however, both the back 

 and the sides are dark bluish grey. A good characteristic is afforded by the 

 pale stripe along the middle of the back, consisting of a single row of 

 tubercles, being of particular distinctness in the young; in the adult it is 

 best seen in the contracted specimens. The foot-sole is of a pure white. 

 The colour of the mucus turns very pale yellow only when the animal is 



Fig. 22. Jaw OÏ Arion circitiitscripfiis, 34/i . Fig. 23. )a.\v oi' Arioi/ circitiiiscn'ptus, ^'^h. 



Fig. 24. 

 Radular teeth oi Arion circumscriptiis, 400'i 



2.0 



1 



Fig. 25. 

 Radular teeth of Arion circitiiiscripiits, 400/i_ 



Strongly irritated ; otherwise it is colourless, an exception, however, being 

 found in the var. nigra, where the mucus on the sides may be milky white. 



The right lateral .shield-band forms a curve on the upper side of the 

 respiratory orifice; where the lateral pigmentation is stronger than usual, 

 such as in var. nigra, the outer limitation of the band is indistinct, but, 

 seen as a whole, at least in Norwegian specimens, this character is one of 

 the best external ones, of especial value in fixed material (fig. i). 



The jaw (fig. 22 — 23). The outlines vary from a proportionally flat 

 arc to a nearly semilunar shape ; the breadth never exceeds i V2 mill. It 

 is yellow to pale brown, as a rule a trifle darker than that of Arion lior- 

 tensis, but is hardly distinguishable from the latter. The slender ribs are 

 often indistinct, especially the lateral ones, in such a way as to make it 

 difficult to fix their number, which, however, may often be from 10 to 12. 



The r adula (fig. 24 — 25) is fi-om 3 to 4 mill. long. It offers no 

 reliable characteristics of the species, hax'ing a strong likeness to that of 



Vid.-Selsk. Skrifter. I. M.-.N. Kl. 1922. No. 5. 3 



