10 NILS ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. V. PROSOBRANCHIA. 1 DIOTOCARDIA. 



1, Zygobranchia : Branchi« 2; both auricles equally developed. 



a. Shell spirally twisted, with a spiral slit; operculum present; radula oo . 1 . 

 (4 , 1 . 4) . 1 . c» ; eyes sessile at the outer base of the tentacles 



Fam. Pleurotomariidce. 



b. Shell limpet or cap-shaped, usually with a short, straight anal slit; no oper- 

 culum ; radula oo . 1 . (4 . 1 . 4) . 1 . oo (except in Propilidium); eyes pedunculate 

 or subsessile at the outer base of the tentacles, or absent (in Propilidium) 



Fam. Fissurellidce. 



2. Azygobranchia : 1 ctenidium (= the left of the Zygobranchia); right auricle 

 blindly closed. Shell spirally twisted; operculum present. 



a. Operculum usually multispiral with central nucleus; animal with epipodial 

 cirri. 



+ . Peristome of the shell incomplete (not continued by a callous margin on 

 the body whorl); shell nacreous; jaws present or not Fam. Trochidce. 

 + +. Peristome continuous; shell not nacreous; jaws present 



Fam. Cydostrematidce. 



b. Operculum jiaucispiral with excentric nucleus; animal without epipodial cirri; 

 peristome incomplete; shell not nacreous; jaws and dentition unknown (sy- 

 stematical position doubtful)' Fam. Adeorbidie. 



Synopsis of Genera and Species. 



Fam. Acmaeidae. 



Aenuea Eschscholtz 1830. 

 Tectura Audouin 1830.- 



Radula with 3 lateral but no median teeth; a branchial plume in the mantle 

 cavity, but no cordon around the foot; eyes on the base of the tentacles. 

 I. Radula with no uncinus: 0.2. (1.0.1). 2.0; teeth of the radula blunt (subg. 

 Acmaea sensu stricto). 



A. Outer laterals smaller than the inner ones; shell internally with a dark brown 

 centre of regular form within the mantle scars, and with a broad, continuous, 

 dark brown border; externally sculptured with fine radiating striae; colour: 

 brown stripes or spots, forming a net-like texture, sometimes uniformly white 

 or brown. L. 20—30 mm A. testudinalis (Müller 1776). 



PI. 1 figs. 1—7; PI. 6 fig. 1. 



B. Lateral teetli of the radula uniform in size. Small forms. 



1. Shell li^^ht yellowish with radiating stripes of red (rarely unicoloured) ; 

 sculpture consisting of obsolete radiating lines. L. 7 — 14 mm 



A. virginea (Müller 1776). 



PI. 1 figs. 9—15. 



^ G. 0. Sars (1878) places these forms under fam. Solariidie. 

 ^ For this synonym confer Watsox (1886). 



