30 NILS ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. V. PROSOBRANCHIA. 1 DIOTOCARDIA. 
fms, shingles, 1 sh., 1. 20.3 (v. Yhlen). — NW. off Siggen, Bergen, 100 fms, 3 shs, 
max. 1. 47 (1875). — NNW. off Bergen, 90—150 fms, fish bank, many shs, max. l. 
53 (Olsson). D:o, 90—170 fms, 3 shs, max. 1. 42.5 (Uddström). — Bergen, 4 shs, max. l. 
28.5 (M. Sars). — Trondhjem, 3 shs, max. 1. 47 (Sundevall). — Norvay, without de- 
finite locality, many shs, max. I. 52 (Rasch), Fig. 24. 
Shetland: Baltasound, on stones, tide mark, 2 sps, max. 1. 23 (Hammar- 
sten 1909). 
Varieties: 
Shell with coarse ribs, type, Fig. 26; with fine but sharp ribs, var. depressa 
PENNANT 1777, Fig. 24; depressed; fine ribs; inside dark bluish, var. coerulea LINNÉ 
1767 (in JEFFREYS 1865), Fig. 25. 
Limits of formal variation: 
| I. | 14—18 | 22—26. | 30—34 | 38—491 | 46—50, | 54,58 
| Scandinavia; De 10—153 16.8—23 | 24—30 30—38.4 31—45 46—50 
| 75 sps h. | 3.5—8 | 5.5—10.3[(7)'9—16 | 13—19 | 15—25 | 20—28 
| measured apex | 4.5—7 | 7.2—10.2/10.8—14.4|13.8—18 |10.4—21.6| 17—26.8| 
General Distribution: 
Norway to Lofoten, 1. 39 (G. O. Sars 1878); to Vanö, 70” N. (Schneider 1895). 
Nordlanden (Krause 1887; Lovén 1846). — West coast (Friele 1874; Lilljeborg 1851). 
— Christianiafjord (Asbjörnsen 1854). — Bohuslän (Lovén 1846; Malm 1835). — 
Denmark, Bulbjerg, dead sh. (Mörch 1871); west coast of Jutland (Christensen 1907). — 
Faroes 1, 60 (Mörch 1868). — British Isles (Jeffreys 1865). — Dogger Bank (Har- 
greaves 1910). — Holland and Belgium, rare (Maitland 1897). — France, very com- 
mon (Locard 1892). — Portugal, abundant (Nobre 1905). — Mediterranean (Carus 
1889). — Cape Verde TIsles (Jeffreys 1865). — Azores (Dautzenberg 1889). — Tide 
mark, in deeper water dead shells. 
Helcion pellucidum (LINNÉ 1758). 
Pl. 1 figs. 27—32: 
Habitat: 
West coast of Sweden: Kullen (LILLJEBORG 1852"), many shs, max. 1. 16; 
br. 12; h. 4.8 (Mus. U.); Skåne, off Hallands Väderö, NW. to N. of Torekov, 8 fms, 
! Exceptional, 1 sp. from Väderöarna. 
