KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 50. N:0 5. 53 



terial. The specimen from Iceland, by its more prodiiced spire, resembles M. prodita 

 most closely, but it has a sliorter aperture and tlie mantle is reflected över the shell mar- 

 gins only, as in M. micromphala. On the otlier hand, the specimen from Spitzbergen is 

 Hke M. micromphala in its shell, but the mantle is somewhat wider (in Fig. 9 it is some- 

 what retracted), while a specimen of the same shell form (from Hammerfest, Fig. 8) has 

 merely its margins covered by the mantle. Aurivillius (1887) describes the one speci- 

 men from Vega St. 1007 as M. groenlandica: the shell has an elevated spire and the mar- 

 gins concealed by the mantle; the otlier specimen he names M. micromphala becaiise of 

 the shell form, in spite of the great extent of the mantle, which is equal to tliat of the 

 E. Greenland specimens (Fig. 11), though here the spire (at least in one specimen) is ele- 

 vated as in M. groenlandica. Finally one specimen from Vega St. 1022 is remarkable 

 for the shell characters being those of M. groenlandica, while the mantle leaves one 

 third of the shell uncovered. 



From the radula, too, no distinguishing characters are obtainable, The specimen 

 from Bremer Sound has in the median tooth one bifid denticle on each side of the central 

 cusp; the laterals and both uncini have each a strong denticle in the inner and a smaller 

 one in the outer margin. In the specimen from Hammerfest a smaller bifid denticle is 

 seen outside the inner bifid one in the median teeth; two others are present in the outer 

 margin of the laterals and two in the inner margin of the inner uncini; outer uncini smooth. 

 The Iceland form has two distinct denticles on each side of the median cusp (the outer 

 one smaller, one in each margin of the lateral tooth and one in the inner margin of the 

 inner uncinus; outer uncinus smooth. The radula of one E. Greenland specimen, lastly, 

 shows one denticle on each side in the median tooth. G. O. Särs (1878) figures in M. pro- 

 dita 3 distinct cusps on each side of the median tooth, one in each margin of the laterals 

 and one in the inner margin of both uncini. Bergh (1887) describes for the same form 

 2 — 3 side-denticles in the median tooth, 1 — 2 stronger denticles in the inner and 2 smaller 

 ones in the outer margin of the laterals; 1 or rarely 2 denticles in the inner margin of the 

 inner uncinus and the same occasionally in the outer uncinus, which is generally smooth. 



Thus the radula too, just like the mantle and the shell, presents differences within 

 limited localities of separate districts. These differences are evidently due to variation; 

 and the assumption that all the different forms belong to one and the same species is 

 more acceptable, also from a geographical point of view, than a division into separate 

 species. 



The Aiiatoiny of the Geiiital Apparatus. 



Bergh (1886 — 87) has only very briefly described the anatomy of the genital 

 organs in M. prodita (and glabra), which he examined by dissection, a method always 

 giving uncertain results in small objects. As this genus is of great importance for 

 the knowledge of the reciprocal relations of the whole group, I have considered it 

 necessary to examine a specimen microscopically from sections and to reconstruct from 

 these the genital apparatus. The figure thus obtained (Text Fig. 1) is quite different 

 from that given in Bergh's main descriptions. 



