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 laterål plate is very 
variable. Møbius records (18) thåt:in | LL. rudis var. tenebrosa it 
varies: from 3 to 6. In this form I have found 5; denticles mm two 
specimens. from: Sealand. (Fig. 6.a), and 6 in, one specimen from Ice- 
FED i 
SI 6. SE ar lateral tooth of Littorina rudis. se: 
a The vi sa from: Smaalandsbugten, Sealand, The" variety .tenebrosa 
from. Bordeyr, Filer e The Littorina groenlandica from Velthannnedjar: "Teeland 
(= a typical rudis). d The Littorina. g-gromlændiea from Berufjordr, Iceland. 
land (Fig. 6 b). -In two specimens-of--Z. groenlandica from Ice- 
land I have noticed 6 dentieles (Fig. 6 c, d). "These numbers ne- 
gative the third distinctive charaeteristic 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: 
"Range of Variation. … Presumed normal number. 
Central tooth: 3—5 5 
1 lateral tooth  3—6 5 
mee re eS 4 4 
er Re ES REED 6 
It may then be asked — if Littorina tenebrosa, Mont… and 
Latørina groenlandica, Menke, are not distinet.species, are they to 
