Introduetion. 
Since the publication of Part IIT of the present series of monographs, entitled 
Opisthobranchia and Pteropoda, I have been engaged in studies with the view to 
the publication of faunistical accounts of the remaining groups of Northern and Arctic 
Mollusea in the collections -of the Swedish State Museum. The present work forms 
the beginning of a faunistic revision of the Prosobranchia. Based on a very rich 
material, it aims to give in the first place a survey of the variation of these molluscs 
within the Boreo-Arctic region. In general the same principles have been followed as in 
the earlier part; thus in the first section a scheme is given for the examination of the 
species, arranged according to the commonly adopted classification, and in the second 
section the distribution of the forms has been stated, both according to the collec- 
tions (under the heading »Habitat») and to the Literature (under »General distribu- 
tion»). The descriptions of the new forms are placed together in a separate section 
at the end. The list of Literature has reference to all the authors cited, and the 
Index ineludes all the names mentioned in the work. On the Plates are figured 
(photographs) all the species included in the collections, representing different local- 
ities and different forms, to show their geographical, formal and sculptural variation. 
The same principles have been followed to show the variation of the radula in the 
polymorphous genera Margarita and WSolariella. ; 
In order to get an objective survey of the variation I have tried to express it 
mathematically, but this has been limited to the formal variation, as that lends itself 
best for expression by numbers, obtained by taking measurements of the shells. I 
have studied the variation of the specimens from different districts separately, by 
which method it is possible to compare the faunistic provinces with one another as 
to the character of the variation. I have preferred this way of dealing with the 
material, i. e. from a zoogeographical point of view, considering it to be of greater 
importance where Fauna is concerned, though recognizing that an analytical study 
of the species with descriptions of varieties based on morphological characteristics 
would be of a more purely systematic value. 
The general method of expressing variation mathematically is to draw up a 
curve showing how the character under consideration varies around a median value 
