NILS HJ. ODHNER, PTISANULA LIMNiEOIDES. 3 



are observable on the middle-part of the whorls. No special 

 apical sculpture is present. 



Measurements in millimetres. The largest specimen from 

 Bel Sound (Fig. 2): Height 2.7; breadth l.e; h. of aperture 

 1.3; br. of ap. 0.7; h. of last whorl 2.i; number of whorls 

 3 2 /3. The specimen from Horn Sound (Fig. 3): H. 2.6 ; br. 

 1.5; h. of ap. 1.4; br. of ap. 0.7; h. of last whorl 2; number 

 of whorls 3 Va. The largest specimen from Kapellbacken 

 (Fig. 4): H. 3.i; br. 1.7; h. of ap. 1.8 ; br. of ap. O.9; h. of 

 last whorl 2.4; number of whorls 4; another shell (Fig. 1): 

 H. 3; br. 1.5; h. of ap. 1.5; br. of ap. O.7; h. of last whorl 

 2.3; number of whorls 3 3 /4. 



Variation of the Shell. 1 The dimensions given above 

 and the Figures 1 — 4 denote a formål' variation, that finds 

 expression in a stretched or a somewhat inflated body-whorl, 

 owing to which the aperture is lengthened or somewhat wider 

 while the umbilicus is narrow or more conspicuous. Such 

 variations are present in both the fossil and the recent shells. 

 Besides this, there seems to be a constant difference between 

 the recent form from Bel Sound and the fossil shells, the 

 apical whorls of the first-named being somewhat more de- 

 pressed and broad, those of the last-named comparatively 

 high and narrow. The post-nuclear whorls are a little higher 

 and broader in the recent shells than in the fossil ones. 

 This circumstance produces a somewhat smaller size in the 

 fossil shell, compared with a recent one with the same number 

 of whorls. The specimen from Horn Sound approaches in 

 this respect the fossil ones. In the sculpture no difference 

 can be observed, the fossil shells too exhibiting indistinct 

 opaque spiral lines and fine lines of increase; in otber parts 

 they are smooth. 



The difference in size named is too slight to be considered 

 as a phenomenon of dwarfing of the fossil specimens, such as 

 I have shown to exist in other forms, e. g. Margarita helicina 

 and Natica dansa from Bohuslän and in Velutina undata from 

 Finmark, 2 as well as in terrestrial molluscs, e. g. Pupa arctica 

 in Sweden. 3 It leaves, however, the question open, whether 



1 The Description of the shell and its variation has been published 

 in Geologiska Föreningens i Stockholm Förhandlingar Bd 35, H. 5, Maj 1913. 



2 Odhner 1912 and 1913. 



3 Odhner 1910. See list of literature. 



