RISSOID^. 2fi3 



outer one laterally produced, the inner wide, callous ; columella 

 solid. 



Subfamily BISSOIN^. 



Shell ovate or elongated. Operculum paucispiral, not provided 

 with a process. Foot of animal without lateral processes. Size 

 small. Marine. 



RissoA, Fremenville. 



Named after the French zoologist, Risso. 



Syn. — E-issostomia, Sars. 



Distr. — About 15 sp. Universally distributed, but most 

 abundant in the north temperate zone. R. costulata, Risso 

 (Ixxi, 28). 



Shell minute, white or horny ; conical, pointed, many-whorled ; 

 smooth, ribbed, or cancellated; aperture rounded; peristome 

 entire, continuous ; outer lip slightly expanded and thickened ; 

 operculum subspiral. 



The animal has long, slender tentacles, with eyes on small 

 prominences near their outer bases ; the foot is pointed behind ; 

 the operculigerous lobe has a wing-like process and a filament 

 (cirrus) on each side. Lingual teeth single, subquadrate, hooked, 

 dentated ; uncini 3 ; 1 dentated, 2, 3, claw-shaped. The}^ range 

 from high-water to 100 fathoms, but abound most in shallow 

 water, near shore, on beds of fiicus and zostera. 



Rissoa is active and bold, floats like its congeners, and spins 

 a byssal thread instantaneously on being detached from a 

 crawling position. The incessant play of the cilia that fringe 

 the tentacles is ver}^ striking ; it appears to be caused by the 

 action of a double row of muscles in each tentacle, arranged in 

 the form of a siphon, which is perceptible through the trans- 

 parency of the integument. The pallial filaments probably serve 

 the purpose of supplementary tentacles to warn the animal of 

 impending danger. 



MANZONiA, Brusina. Peristome duplicate. There are 8 Euro- 

 pean species. 



ANABATHR.0N, Fraueufeld. Shell very small, thick, oblong, 

 angulate, scalariform, imperforate, smooth; aperture rounded, 

 peritreme continuous. Operculum corneous. Australia. B. 

 contabulata,¥YSiVienf. (Ixxii, 40). 



PLAGIOSTYLA. Fischer. Shell transparent, apex papillary, last 

 whorl descending, aperture semilunar, pillar-lip oblique, recti- 

 linear. Europe. 



zippoRA, Leach. (Acme, H. and A. Adams.) Shell subcylin- 

 drical, smooth or longitudinally ribbed ; margin of aperture 

 reflected. B. Moutonii^ Dupuy (Ixxii, 41). 



PTEROSTOMA, Deshayes, 1864. Shell elongated, turriculated ; 

 peristome continuous, circular, very dilated and margined ; colu- 



