10 NILS ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. V. PROSOBRANCHIA. 1 DIOTOCARDIA. 
1. Zygobranchia: Branchie 2; both auricles equally developed. 
a. Shell spirally twisted, with a spiral slit; operculum present; radula o.l. 
(4.1.4).1. &; eyes sessile at the outer base of the tentacles' 
Pam. Pleurotomariide. 
b. Shell limpet or cap-shaped, usually with a short, straight anal slit; no oper- 
culum ; radula &.1.(4.1.4).1.0 (except in Propilidium); eyes pedunculate 
or subsessile at the outer base of the tentacles, or absent (in Propilidium) 
Fam. Fissuwrellide. 
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 ineomplete (not continued by a callous margin on 
the body whorl); shell nacreous; jaws present or not Fam. Trochide. 
++. Peristome continuous; shell not nacreous; jaws present 
Fam. Cyclostrematide. 
b. Operculum paucispiral with excentric nucleus; animal without epipodial cirri; 
peristome incomplete; shell not nacreous; jaws and dentition unknown (sy- 
stematical position doubtful)' Fam. Adeorbidee. 
Synopsis of Genera and Species. 
Fam. Acmeeide. 
Acmea ESsCcHSCHOLTZ 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 uneimus: 0.2.(1:0. 1):2-: 0; teeth of the radwajbluntt(sobat 
Acmea 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 strige; colour: 
brown stripes or spots, forming a net-like texture, sometimes uniformly white 
or brown. L. 20—30 mm A. testudinalis (MÖLLER 1776). 
Pl1 figs) 1-7 RING TNS SE 
B. Lateral teeth of the radula uniform in size. Small forms. 
1. Shell light yellowish with radiating stripes of red (rarely unicoloured); 
sculpture consisting of obsolete radiating lines. L. 7—14 mm 
A. virginea (MULLER 1776). 
Pl. 1 figs. 9—15. 
GEO SARS (1878) places these forms under fam. Solariide. 
? For this synonym confer WATsoNn (1886). 
