KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 50. N:0 5. 



21 



The coloiir in high arctic shells is usually brownish or greenisli, in the smaller south- 

 ern forms uniformly påle yellowish or wliite, as in all young shells. Darker and lighter 

 zones usually appear in arctic specimens (cfr, Figs. 10 and. 16); var. w^toto Jeffreys 1877 

 has two distant purplish bands, var. janthostoma Deshayes 1841 is irregularly white- 

 zoned with the aperture interiorly violet, var. zonata Middendorfp 1851 has a reddish- 

 brown tint with a zone of Ii vid white at the base. 



G, O. Särs (1878) considers the small southern white or semipellucid form as a 

 separate species, N. affinis Gmelin. The characters of the shell however are not of such 

 an importance as to justify a specific distinction; moreover N. affinis of Särs must be 

 considered as a geographic (southern) variety, which finds in greater depths the favour- 

 able conditions for existence and here becomes paler-coloured and smaller-sized. The 

 characters of the radula (larger denticles of the median tooth) are subject to variation. 

 The number of the whorls, compared with that of arctic forms of the same size, shows 

 that it is a dwarf form, while on the other hand the large specimens from Siberia and 

 Behring Sea, with about the same number of whorls, must be considered as a large-sized 

 race of N. clausa. 



The number of whorls it is very difficult to determine exactly in older shells, be- 

 cause the first whorls are nearly always worn away. Thiis the tables above, exhibiting 

 the numbers really observed, only give a vague idea of the great difference in size be- 

 tween forms with the same number of whorls within separate districts. To get a survey of 

 this matter it is necessary to start from a very young shell, where all the whorls remain, 

 and from that stage follow the growth of the species at the same locality by comparing 

 larger specimens and thiis finding out the namber of whorls in successive stages. The 

 following table gives a survey of the relation between size and whorls or the rapidity of 

 increase in different districts: 



N u m b o r o f w h o r 1 s : 



Height of the shell from 



Skagerrack and Koster 



Kära Sea 



Siberia, Behring Sea - 



Treurenbnrg Bay 



King Charles Land ; Hope Island . 



Berufjord 



Godhavn 



Skinderh valen; Fiske rnyss . . . . | 

 Arsukfjord . . . . J 



2 V, 



3 



3 '/, 



4 



4'/2 



5 



5 V. 



1.7 



2.7 



3.9 



5.2 



about 7 



9 (max.) 





— 



— 



4 



8 



13 



20 



28.5(max). 



— 



4.2 



7.9 



10 



13 



21 



29 



2 



— 



4.5 



7.2 



11.5 



16.2 



23—29 



— 



about 3 



4.5 



— 



7.2 



11.5 



— 



— 



2.9 



about 4.5 



5.3 



9.2 



12.2 



16 



— 



3.5 



5 



8 



13.5 



17—20 



30 



— 



— 



4,3 



7.5 



10 



14.5 



— 



about 42 

 [36, Bel Sound] 



A specimen with 5 whorls may consequently vary from 21 to 9 mm in height; tliese 

 extreme values are met with in the Behring Sea on the one side and in southern Scandi- 

 navia on the other; and they are connected by numerous intermediate values, e. g. inlce- 

 land. In Spitzbergen we find a larger size in shallow water (Treurenburg Bay); a smaller 

 size in specimens from King Charles Land, and deep-water forms in general show similar 



