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NILS ODHNEB, NORTHERN AND ARCTIC INVERTEBRATES. III. OPTSTTIOBRANCHIA ETC. 



Synopsis of Genera and Species. 

 Fam. Actaeonidae. 



Actaeon Montfort 1810. 



Shell spirally striated; whorls 7 — 8; columella with a prominent fold; colour 

 reddish or violet, often with 3 white spiral girdles. The cephalic disc reflexible over 

 the anterior part of the shell. L. about 17 mm. A. tornatilis (Linne 1766). 



Shell pellucid, greenish and more shining than in the type, with 4 whorls 

 and yellow spiral bands. L. 3,8 mm. var. tenellus Loven 1846. 



Fam. Tornatinidae. 



I. Shell cylindrical or conical; spire obtuse Retusa Brown. 



II. Shell contracted at both ends; spire acuminated Volvula Adams. 



Retusa Brown 1827. 

 Utriculus Brown 1844. 1 



I. Spire concealed. 



A. Apex abruptly truncated. 



1. The last whorl not tapering from the middle upwards; no spiral stria?; 

 outer lip projecting beyond the apex; shell constricted in the middle: 

 colour white. L. 4 mm. R. truncatula (Bruguiere 1792). 

 Shell more transparent, nearly cylindrical, not constricted in the middle. 

 L. 5 mm. var. pellucida (Brown 1827). 



2. The last whorl tapering backwards, abruptly conical : spiral stria? in- 

 distinct ; outer lip projecting beyond the apex and furnished with a sharp 

 keel surrounding the sunken apex. L. 8 mm. R. ovata (Jeffreys 1870) 



= conulus (Deshayes) in G. O. Sars 1878. 



B. Apex obtusely rounded. 



1. Shell umbilicated and spirally striated; columella folded. 



a. The breadth of the shell = the half length. 



+ . Apex deeply and widely perforated; colour white. L. 4 mm. 



R. umbilicata (Montagu 1803). 

 + +. Apical perforation smaller; columellar fold and stria? very distinct; 

 colour white, epidermis brown. L. 2 mm. 



R. strigella (Loven 1846). 



b. The breadth — : \U of the length: shell with a few coarse spiral ridces and microscopical striae. 

 L. 2,9 mm. R. substriata (Jeffreys 1877). 



1 The name Utriculus was employed by Schumacher in 1817 tor a Prose-branch. 



