RISSOIDA. 263 
outer one laterally produced, the inner wide, callous; columella 
solid. 
Susramity RISSOINAL. 
Shell ovate or elongated. Operculum paucispiral, not provided 
with a process. Foot of animal without lateral processes. Size 
small. Marine. 
Rissoa, Frémenville. 
Named after the French zoologist, Risso. 
Syn.—Rissostomia, Sars. 
Disitr—About 75 sp. Universally distributed, but most 
abundant in the north temperate zone. 2. 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. They range 
from high-water to 100 fathoms, but abound most in shallow 
water, near shore, on beds of fucus 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 very 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. 
ANABATHRON, Frauenfeld. Shell very small, thick, oblong, 
angulate, scalariform, imperforate, smooth; aperture rounded, 
peritreme continuous. Operculum corneous. Australia. R. 
contabulata, Frauenf. (1xxii, 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. &. Moutonii, Dupuy (Ixxii, 41). 
PTEROSTOMA, Deshayes, 1864. Shell elongated, turriculated ; 
peristome continuous, circular, very dilated and margined ; colu- 
