66 NILS ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. V. PROSOBRANCHIA . 1 DIOTOCARDIA. 



and with a more rounded base (indistinct 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- 8 in number; on the apical whorls 

 2 or more equally prominent ridges appear simultaneously; there are no prominent 

 aiigular 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 (cfr Gould & Binney 

 1870), while in forma multilirata it is coarsely sculptured. The typical var. grandis 

 Mörch is figured on PI. 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.) 1 am unable to de- 

 finitely say, but the M. cinerea var. striata illustrated by Sars, in Moll. reg. Arct. 

 Pi. 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 

 lirse appear simultaneously on the 3rd whorl, but a very feeble trace of a keel may 

 be visible even on the 2nd whorl. One of the earlier lirte 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 typica 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, 



