38 NILS ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. III. OPISTHOBRANCHIA ETC. 



Pteropoda. 



Heart, nervous and yenerative systems as in Opisthobranchia ; foot with wing- 

 shaped processes, homologous ■with the parapodia in Tectibrmichia. 



vSynopsis of Faniilies. 



Subordo 1 Thecosomata. 



Maiitle, mantle-cavity and shell present; liead indistinct; one pair of tentacles; 

 f in-lobes continuous över the moutli ; radula 1.1.1. 



I. Shell calcareous, very thin. sinistral, with operculum; branchial cavity dorsal; 



colnmellar mnscle ventral; anus on the right side Fam. Limacinidce. 



II. Shell symmetrical, without operculum; branchial cavity ventral; columellar muscle 



dorsal; anus on the left side Fam. Cavoliniidce. 



Subordo II Gymnosomata. 



Mantle. niantle-cavity and shell absent; head distinct; two pairs of tentacles; 

 f in-lobes separated; anus on the right side. 



Body long, acuminated behind; no gills Fam. Clionidce. 



Synopsis of Genera and Species. 



Fam. Limacinidae. 



I. Shell depressed, broadly umbilicated, with transversal strise Limacina Cuvier. 

 II. Shell conical, narrowly umbilicated, sniooth or spirally striated 



Spirialis Eydoux & Souleyet. 



Liiuiiciiiii OuviER 1817. 



Whorls 5 — 6, transversally striated; spire somewhat elevated; columella straight 

 with reflexed margin; umbilicus surrounded by a keel; outer aperture-margin semi- 

 circular. Shell hyaline; animal dark-purplish or violet with light, transparent wings. 

 Diameter of the shell about (^ mm. L. behcina (Phipps 1774) 



= arctica (Fabeicius 1780). 



