12 NILS ODHNER, NOKTHERN AND ARCTIC INVERTEBRATES. V. PROSOBRANCHIA. 1 DIOTOCARDIA. 



HelciOll MONTFORT 1810. 



Surface of the shell smooth, with only a few radiating stria?. Colour brownish 

 or greenish with broken rays of a bright blue. L. 15 — 20(24) mm 



H. pellucidum (Linne 1758) (=-Nacella pellucida Auct,). 



PI. 1 figs. 27—32. 



Fam. Lepetidae. 



I. Uncini of the radula smooth in the margin; central tootli 5-cuspidate; apex of 

 the shell erect; surface finely granulate Lepeta Gray 1847. 



11. 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 Pilidivnn Forbes 1849 



(= Scutellina G. 0. Sars 1878). 



Lepeta Gray 1847. 



Shell depressed conic with subcentral apex; hinder slope usually straight; sur- 

 face latticed by fine radiating and concentric strise, granulated in their crossings; 

 colour whitish, brownish or reddish. L. 10—18,7 mm L. coeca (Müller 1776). 



PI. 2 figs. 2—17. 



Cryptobniiicliia Middendorff 1851. 



Shell solid, smooth or with very fine strife; apex anterior or subcentral ; hinder 

 slope of the shell convex; mouth ovoid, broader posteriorly; colour white or grayish. 

 L. 10—20 mm C. alba Dall 1869. 



PI. 2 fig. 1. 



Pilidimn Forbes 1849. 



Shell depressed with fine granular radiating ridges; apex anterior; hinder 

 slope arcuated; mouth oval; colour orange or reddish. L. 7 — 9.8 mm 



P. fulvum (Müller 1776). 



PI. 2 figs. 18—23. 



Colour whitish • var. albula Jeffreys 1865. 



PI. 2 fig. 23. 



Shell large, much dejjressed and broad in proportion to the length 



var. expansa Jeffreys 1865. 



PI. 2 figs. 18, 19. 



Fam. Pleurotomariidse. 



Scissurella d'Orbigny 1823. 



Anal slit open; operculum horny, circular, multispiral with central nucleus; 

 animal with 2 long epipodial filaments on each side. 



