80 NILS ODHNER, NOBTHERN AND ARCTIC INVERTEBRATES. V. PROSOBRANCHIA . 1 DIOTOCARDIA. 



margin; surface for the most part smooth, but the umbiKcal region finely striated 

 by lines emerging from the umbilicus and running arcuated towards the peristome, 

 the upper nepionic whorl furnished in the middle with 2 faint spiral lirse, becoming 

 subsutural on the next whorl and disappearing. Height l.u; diameter 1.8; height of 

 aperture 1 mm ; whorls 4. 



Operculum corneous with central nucleus and 7 spirals. 



Radula with about 50 rows, of the formula 35 . . (4 , 1 . 4) . . 35. The central 

 tooth very large, its greatest breadth in, or just below, the middle, its upper margin 

 not markedly denticulated. Median teeth narrow, high, slightly pinched below the 

 apex, with about 4 marginal denticles. No transition (lateral) tooth. Lamellae with 

 a spoon-like recurved apex, the inner ones denticulated on both sides, further out 

 smooth, the outmost very long and narrow, pin-like. 



Animal without eyes but with eye-peduncles preserved. The intestine is situated 

 quite in the distal part of the last whorl and the stomach behind it. The content 

 of the stomach and of the intestine was mud, containing Foraminifera, Diatomacea 

 etc. The otocysts, situated close behind the pedal ganglia, contain many otoliths. 



By its nepionic sculpture this form is distinct from other species of Cyclostrema. 

 In C. basistriatum a sculpture of this kind sometimes occurs, but it is not constant. 

 The tumid whorls and the elevated spire are in such cases reliable characters for the 

 determination of C. turgidum. For the distribution see pag. 77. 



