12 NILS ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. V. PROSOBRANCHIA. 1 DIOTOCARDIA. 
Helcion MONTFORT 1810. 
Surface of the shell smooth, with only a few radiating strig. Colour brownish 
or greenish with broken rays of a bright blue. L. 15—20(24) mm 
H. pellucidum (LINNÉ 1758) (= Nacella pellucida Auct.). 
Pl. 1 figs. 27—32. 
Fam. Lepetidee. 
I. Uncini of the radula smooth in the margin; central tooth 5-cuspidate; apex of 
the shell erect; surface finely granulate Lepeta GRAY 1847. 
II. Uncini of the radula finely ciliated in the margin; central tooth 3-cuspidate; 
apex of the shell inclined forward. 
A. Surface smooth or with concentric lines only; colour whitish. Cusps of the 
central tooth nearly equal in size Cryptobranchia MIDDENDORFF 1851. 
B. Surface with radiating ribs and granula; colour orange, reddish or grayish. 
Middle cusp of the central tooth very large Pilidium FORBES 1849 
(= Scutellina G. O. SArs 1878). 
Lepeta Gray 1847. 
Shell depressed conic with subceentral apex; hinder slope usually straight; sur- 
face latticed by fine radiating and concentric strie, granulated in their crossings; 
colour whitish, brownish or reddish. LL. 10—18.7 mm L. coeca (MULLER 1776). 
Pl. 2 figs. 2—17. 
Cryptobranchia MIDDENDORFF 1851. 
Shell solid, smooth or with very fine strige; apex anterior or subcentral; hinder 
slope of the shell convex; mouth ovoid, broader posteriorly; colour white or grayish. 
L. 10—20 mm C. alba DaALL 1869. 
BI2rje I. 
Pilidium FORBES 1849. 
Shell depressed with fine granular radiating ridges; apex anterior; hinder 
slope arcuated; mouth oval; colour orange or reddish. &L. 7—9.s mm 
P. fulvum (MUEEER 1716). 
Pl. 2 figs. 18—23. 
Colour whitish var. albula JEFFREYS 1865. 
Pl2rfg. 23. 
Shell large, much depressed and broad in proportion to the length 
var. expansa JEFFREYS 1865. 
Pl. 2 figs. 18, 19. 
Fam. Pleurotomariideae. 
Secissurella D'ÖRBIGNY 1823. 
Anal slit open; operculum horny, circular, multispiral with central nucleus; 
animal with 2 long epipodial filaments on each side. 
