301 
be regarded as well defined varieties of L. rudis , or are they onlv syno- 
nymous for that species? Witli regard to L. tenebrosa it is evi¬ 
dent that it deserves to rank as a variety. Besides its small size 
it is further distinguished by the faet that the shell is smootli or 
indistinetly ribbed, dusky, and very often tesselated or chequered 
(Conf. Montagu 4; Jeffreys 14). It can also be added that the 
normal number of denticles on the first lateral tooth is 4, whereas 
in the typical forms of rudis it seems to be 5. It is a brackish- 
water form whose size — as is the case with the common forms of 
rudis — gets gradnally smaller as the salinity of the water de- 
* 
creases. In the following table there are given some examples of 
the reduction in size of L. rudis from the northern Kattegat to the 
Sound. 
The size recorded is the average-size of the largest specimens, 
the largest one being taken from every 25 specimens out of any 
larger number. 
Locality. 
Salinity. 
Size. 
Number of specimens 
examined. 
Frederikshavn, north. Kattegat 
ro 
O 
O " 
18,5 mm 
160 
Hellebæk, Southern Kattegat 
1,5 - 
12 - 
135 
Trekroner, the Sound 
1,2 - 
10 - 
170 
The name Littorina groenlandica was applied by Menke to 
specimens of L. rudis from Greenland, and by other authors to L. 
rudis from Iceland, Norway, Spitzbergen, etc. Still, no characteristic 
peculiar to the arctic forms of rudis in opposition to those from 
other localities has been pointed out. G. O. Sars was of 
opinion (20) that groenlandica might be distinguished as a variety 
of great size, and he records that the common rudis is 10 mm in 
lenght, whereas groenlandica is 20. No such difference in size, 
however, exists. The size of L. rudis varies in Denmark from about 
8 to 22 mm , the size of L. groenlandica in Iceland 1 ) from 8 to 
1 ) A monstrous specimen of 24 m m ’ s length from Iceland is recorded by 
Mørch (11). 
