298 
determine, except on the central tooth, the exact number of denticles. 
In 12 specimens I found that the number of denticles on the first and 
second lateral tooth was invariably four, 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)!). 
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 mn dula are according to Teilman-Friis (28) as 
tooth of Litt 
littorea L. te follows : 
Em 
Fig. 4. SELdn teeth of SRSSÅ rudis… 240/,, 
a The variety tenebrosa from Smaalands n, Sealand. b The so called Littorina 
groenlandica from Berufjårdr, Iceland. cc The typical rudis 
Littorina rudis Littorina groenlandica 
1) The central tooth has 3 den- 1) The central tooth has 5 den- 
ticles. tieles. 
2) The first lateral tooth has 4 | 2) The first lateral tooth has 6 
denticles. denticles. 
3) The third lateral tooth has 5 3). The third lateral tooth has. 6 
denticles. denticles. 
1) In a very interesting paper H. C. Bumpus (29) has shown that the 
ittorea, 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, ete., than in England. 
