80 NILS ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. V. PROSOBRANCHIA. 1 DIOTOCARDIA. 
margin; surface for the most part smooth, but the umbilical 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 lire, becoming 
subsutural on the next whorl and disappearing. Height 1.s; diameter 1.s; height of 
aperture I mm; whorls 4. 
Operculum corneous with central nucleus and 7 spirals. 
Radula with about 50 rows, of the formula 35.0.(4.1.4).0.35. 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 bkelow the 
apex, with about 4 marginal denticles. No transition (lateral) tooth. Lamelle 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 1s 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. 
