On tké Variations observable in some Northern 
species of Littorina. 
By 
A. C. Johansen . 
In tbe genus Littorina it is, as is generally tlie case, a diffi- 
cult. matter to determine what the destinguishing characteristics of 
the species are that would meet with general acceptance. Most 
authors have estimated the range of variation within specific limits 
differently, and not many of the alleged species of Northern Litto- 
rinæ have been unanimously accepted. 
The forms with which I am about to deal come partly from 
the coasts of Iceland, and partly from the seas between the North- 
Sea and the Baltic. In my opinion they can all be referred to the 
three well-known species: Littorina littorea, Linné, L.rudis, Maton, 
and L. obtusata, Linné. 
G. O. Sars has separated L. arctica, Møller, from L. obtusata, 
mainlv on the ground of a difference in the number of denticles on 
the individual teeth, and in the same manner Te ilman-Friis 
has separated L. groenlandica, Menke, and L. tenebrosa, Montagu, 
from L. rudis , Maton, and L. parva, Mørch, from L. littorea, Linné. 
The differences pointed out by Teilman-Friis (27) be¬ 
tween Littorina littorea and Littorina parva are as follows: 
Littorina littorea 
1) The central tooth has 3 den¬ 
ticles. 
2) The radula is proportionally 
short. 
Littorina parva 
1) The central tooth has 5 den¬ 
ticles. 
2) The radula is proportionally 
long, a third part longer than 
that of L. littorea. 
