300 
cord of the number it will be seen that this character can not be 
used to divide the three forms. 
The number of denticles on the third lateral plate is very 
variable. Møbius records (18) that in L. rudis var. tenebrosa it 
varies from 3 to 6. In this form I have found 5 denticles in two 
specimens from Sealand (Fig. 6 a), and 6 in one specimen from Ice- 
Fig. 6. Third lateral tooth of Littorina rudis. 240 /i. 
a The variety tenebrosa from Smaalandsbugten, Sealand. b The variety tenebrosa 
from Bordeyri, Iceland. c The Littorina groenlandica from Yestmannaeyjar, Iceland 
(= a typical ruclis). d The Littorina groenlandica from Berufjordr, Iceland- 
land (Fig. 6 b). In two specimens of L. groenlandica from Ice¬ 
land I have noticed 6 denticles (Fig. 6 c, d). These numbers ne¬ 
gative the third distinctive characteristic given by Teilman-Friis. 
If the number of denticles recorded by Møbius and Teilman- 
Friis, both of whom have treated the teeth separately, be taken 
into consideration, the following table showing the normal number 
of denticles, and the known range of variation in Littorina rudis 
(including L. groenlandica and L. tenebrosa), can be drawn up: 
Eange of Variation. Presumed normal number. 
Central tooth 
3- 
-5 
O 
O 
l st lateral tooth 
3— 
-6 
5 
2 nd — 
4 
4 
3 rd — 
3—7 
6 
It may then be asked — if Littorina tenebrosa, Mont. and 
Littorina groenlandica, Menke, are not distinet species, are they to 
