66 NILS ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. VV. PROSOBRANCHIA. 1 DIOTOCARDIA. 
and with a more rounded base (indistinet angular keel) has been described by G. O. 
Sars (1878) as var. grandis Mörch, but it is not identical with that form. In the 
other extreme variety of the present species, var. striata Brod. & Sow., the spiral 
ridges are of a uniform prominence, and about 5—38 in number; on the apical whorls 
2 or more equally prominent ridges appear simultaneously; there are no prominent 
angular or umbilical keels; the base is more or less rounded and finely or coarsely 
sculptured. To this form belongs var. grandis forma multilirata Mörch (PI. 4 fig. 34), 
which seems to be very nearly allied to the type of Broderip & Sowerby, because 
of its great number of spiral ridges (about 8 in Sowerby's fig. 3) and its regularly 
rounded base. But in var. striata the base is nearly smooth (cefr Gould & Binney 
1870), while in forma muitilirata it is coarsely sculptured. The typical var. grandis 
Mörch is figured on Pl. 4 fig. 33 and PI. 5 figs. 1—2; the specimens figured have 
been compared with Mörch's originals in Copenhagen and are absolutely identical. 
To this form belongs M. cinerea forma typica of Sars 1878 and var. margaritifera 
Friele 1886. Sars has confused the type and var. grandis, but he gives the latter 
as a synonym of var. striata; Aurivillius (1887), Collin (1887) and Knipowitsch 
(1901) also consider them as identical. Posselt & Jensen (1899) say that M. striata 
is more coarsely spirally sculptured than Mörch's grandis; this agrees with my state- 
ment above, that M. striata is most closely allied to Mörch's forma multilirata. 
Friele (1886) makes the following statement: » Whether M. cinerea var. grandis Mörch 
is identical with the present species (M. striata Brod. & Sow.) I am unable to de- 
finitely say, but the M. cinerea -var. striata illustrated by Sars, in Moll. reg. Arct. 
Pl. XXIV fig. 4 is not M. striata Brod. & Sow.»> 
I have studied the sculpture in different forms and found the following condi- 
tions as to the appearance of sculpture in earlier stages: In the typical form (M. 
cinerea Couthouy as above defined) from Greenland and Finmark, 1 strongly marked 
spiral ridge appears on the 2nd or 3rd whorl accompanied further on, on the 4th or 
5th whorl, by a few fainter ones. The strongly marked ridge may be somewhat 
feebler and the fainter ones may even appear on the 3rd whorl. In forma multilirata 
from Pröven 1 strongly marked ridge appears on the 2nd whorl and some feebler 
ones on the 3rd. In var. grandis from Spitzbergen 2 or more comparatively faint 
lire appear simultaneously on the 3rd wbhorl, but a very feeble trace of a keel may 
be visible even on the 2nd whorl. One of the earlier lire may be more strongly 
marked (Kara Sea). Confer further the notes of sculpture given in several cases 
above. 
In regard to all the formal and sculptural characters, all transitional forms 
occur between forma typieca and var. grandis Mörch, represented in one case by 
forma multilirata and striata, in the other by var. grandis Sars and the form from 
Kara Sea. On this account I consider all these forms as varieties of the same 
species and not, as Friele (1886) does, as distinct species, though the extreme forms 
might be considered to favour such an opinion. 
Variation of the radula: Like the sculpture, the radula also exhibits two differ- 
ent types. In certain forms the rhachian teeth are much dilated towards the sides, 
