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



Pteropoda. 



Heart, nervous and generative systems as in Opisthobranchia ; foot with winy- 

 shaped processes, homologous with the parapodia in Tectibranchia. 



Synopsis of Families. 



Subordo I Thecosomata. 



Mantle, mantle-cavity and shell present; head indistinct; one pair of tentacles; 

 fin-lobes continuous over the mouth; radula 1.1.1. 



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



columellar muscle ventral; anus on the right side Fam. Limacinidee. 



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

 dorsal; anus on the left side Fam. Cavoliniidce. 



Subordo II Gymnosomata. 



Mantle, mantle-cavity and shell absent; head distinct; two pairs of tentacles; 

 fin-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, smooth or spirally striated 



Spinalis Eydoux & Souleyet. 



Limacina Cuvier 1817. 



Whorls 5—6, trans versally 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 6 mm. L. helicina (Phipps 1774) 



= arctica (Fabricitjs 1780). 



