1922. No. 5. 



ARIONIDAE OF NORWAY. 



ribs, too, present variations, and as a maximum there are tbund 18. In 

 the young the lateral ribs are faintly developed and the jaw then often 

 resembles that of Arioii subfiisctis, characterized by 

 the few broad, close-set ribs. \r\ the larger specimens 

 at any rate, some of the ribs are slender, their in- 

 termediate distance being about their own breadth. 



The r adula (fig. 8 — 9) grows to 9 mill. long. 

 A characteristic of the median teeth is the short 

 mesoconus, the length of which is less than the rest 

 of the epithem, and which is not distinctly con- 

 stricted at the base. In any case the larger specimens should be deter- 

 mined by this peculiarity. 



Otherwise it is easily proved that the radula is liable to variation, 

 both as regards the number of teeth, their shape and the speed with 



Fig, 7. jaw of youns 

 Alton atcr, 22 ] 



Fig. 8. Radular teeth of 

 Ar ion atcr, ^^i\. 



Fig. 9. Radular teeth of Arioii ater 

 var. alba clegaiis, ^'^ 1 . 



which the change of figure takes place from the median teeth towards the 

 lateral and marginal ones (compare ^.^. 8 to fig. 9 1. This variation however is 

 greater when not only the Norwegian stock is considered. Thus the magni- 

 ficent monograph of the British slugs (Taylor 1900 — 1907) shows that 

 the inner lateral teeth may present an endocone, a feature which as yet 

 has never been found in Norwegian Arion atcr. 



Abnormities in the shape of the teeth are proportionally frequent, for 

 instance a slight asvmmetri is not uncommon. 



Organs of reproduction (fig. 10 — 12). 



As in other species it is chiefly the lower part of the reproductive 

 organs which presents the most important characters. The ovispcnnatoduct 

 (o.-sp.-d.) divides into an unusually short oviduct (o.) and into a proportionally 

 long vas deferens {v. d.) which passes into the thicker epiphallus {ep). In 

 the adult the rcceptacidiim {rec.) has a large bladder which usually appears 

 like a lean, folded up bag; in rare cases it is found dilated as in fig. 10). 

 The genital retractor iretr.) is a broad, powerful muscle, which separates 

 into a slender branch for the stalk of the receptaculum with attachment 



