298 
determine, except on the central tootli, the exact number of denticles. 
In 12 specimens I found that the number of denticles on the first and 
second lateral tooth was invariably fo ur, whereas in the third plate it 
varied from 4 to 7. The normal number on the 
third lateral tooth seems to be 5 (Fig. 3) l ). 
Littorina rudis , Maton, L. groenlandica, Menke, 
L. tenebrosa , Montagu. 
These three forms are all viviparous and in 
the shell no constant difference has been pointed 
out. The distinctive characteristics in the ra- 
Fig. 3. Third lateral dula are according to Teilman-Friis (28) as 
tooth of Littorina 
littorea L. 160 /i. foliows: 
a 
Fig. 4. Central teeth of Littorina rudis. 240 /i. 
a The variety tenebrosa from Smaalandsbugten, Sealand. b The so called Littorina 
groenlandica from Berufjordr, Iceland. c The typical rudis. 
Littorina rudis 
1) The central tooth has 3 den¬ 
ticles. 
i 
2) The first lateral tooth has 4 
denticles. 
3) The third lateral tooth has 5 
denticles. 
Littorina groenlandica 
1) The central tooth has 5 den¬ 
ticles. 
2) The first lateral tooth has 6 
denticles. 
3) The third lateral tooth has 6 
denticles. 
In a very interesting paper H. C. Bumpus (29) has shown that the 
Littorina littorea , Linné, which is supposed to have been introduced 
to the Atlantic coasts of the United States about fifty years ago, in 
its new environments is liable to a greater variation with regard to 
stature, mode of growth, etc., than in England. 
