301 
be regarded sas well defined varieties of L. rudis, or are they only syno- 
nymous: for that species? "With 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 smooth or 
indistinctly ribbed, dusky, and very often tesselated or, chequered 
(Conf. Montagu 4; Jeffreys 14). It. can also' be-added that the 
normal number of dénticles on the first lateral tooth is 4, whereås 
im'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 tåble 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. 
Number of specimens 
Locality. Salinity. Size. mera 
Frederikshavn, north. Kattegat 2,7%0 1857" 160 
Hellebæk, southern Kattegat 15-— 12 - 135 
Trekroner, the 'Sound fa sag 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. 0. Sars was of 
Opinion (20) that groenlandica might be distinguished as a variety 
of great size, and he records that the common rudis is 107" in 
lenght, whereas groenlandica is 20. No such difference in size, 
however; exists: The size! of £. rudis: varies in Denmark from about 
8:to 29 mm, |fhe: sige of L. groenlandica in Iceland ly from 8 to 
1) A monstrous specimen of 24 mm's length from Iceland is recorded by 
Mørch (11). j 
