KUNGL. SV, VET. AKADEMIENS HANDLINGAR. BAND 50. NIO 5. 



Prosobranchia. 



Subordo II Monotocardia (Pectinibranchia). 



Heart with one auricle; ctenidiuin 1, monopectinate^ (filaments developed on one 

 side oiily), nearly always in its whole leiigth fused w^ith the mantle^; pedal ganglia in 

 most cases distinct. Shell siphonated or not. 



Subdivisions^: 

 A. Tsenioglossa. 



Radiila (with few exceptions) 2.1.1.1.2, uncini sometimes multiplied, 



1. Semiproboscidifera: Snoiit intro versible from the mouth (acrembolic). Osphra- 

 dium bipectinate. Shell usually without canal or sinus. 



a. Animal with pedal cords (Cyprma) or pedal ganglia prolonged far backwards 

 {Trivia arctica); mantle reflexible över the sides of the shell; eyes subpedun- 

 culate on the outer side of the tentacles near their base. Shell with the spire 

 usually concealed by the last whorl; aperture central, narrow, occupying the 

 entire length of the shell, canaliculated at both ends; outer lip folding inwards. 

 No operculum Fam. Cyprceidoe. 



b. Animal with pedal ganglia; mantle reflected or not. Shell heliciform to lam- 

 elliform, never canaliculated. 



I. Am'mal with a large propodium reflected över the head; eyes absent or sub- 

 cutaneous behind the tentacles. Shell solid, heliciform, generally wholly 

 external with a callous columellar lip more or less reflected över the um- 

 bilicus; operculum present Fam. Naticidce. 



1 1 Animal without propodium; eyes pedunculate at the outer base of the 

 tentacles. Shell thin, auriform or lamelliform, external or internal, without 

 callosities; no operculum Fam. LamellariidcE. 



2. Rostrifera: Snout contractile but not intro versible; osphradium usually monopec- 

 tinate. 



3. Proboscidifera: Snout intro versible from the base (pleurembolic). Shell siphonated 

 or not. Osphradium bipectinate. 



^ except in Valvata. 



^ The familise are excluded, except as respects Semiproboscidifera, the subject of the present publication. 



