Introduction. 



Since the publication of Part III of the present series of monographs, entitled 

 Opisthobranchia and Pteropoda, I have been engaged in studies with the view to 

 the pubhcation of faunistical accounts of the remaining groups of Northern and Arctic 

 MoUusca in the collections of the Swedish State IMuseum. 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 includes 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 Solariella, 



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 



