MANUAL OF THE MOLLUSCA. 
[Famity Rissomz] 
Includes LitIoPa (p. 255), RIssomna (p. 256), Rissoa (p. 255), 
and the following additional genera :— 
DiastomA, Deshayes, 1864. 
Type, Melania costellata, Lamarck. 
Shell elongated, turreted ; whorls with varices ; sipanties very 
oblique, semi-lunate, entire; base sinuated, subangular ; pos- 
terior angle acute, detached from the penultimate whorl; lip 
thin, curved; columella concave, depressed, narrow. 
Fossil, 4 species. Eocene. Paris basin. 
AMPHITHALAMUS, P. Carpenter, 1864. 
Type, A. inclusus. West Coast of North America. 
Sheli like Rissoa, nucleus large; aperture with a produced 
lip, suddenly contracted in the adult. 
This genus bears the same relation to Rissoa that Stoastoma 
does to Helicina. 
Kerrnostoma, Deshayes, 1848. 
Type, Melania marginata, Lamk. 
Shell elongated, turriculated, regularly conical; generally 
striated transversely. Aperture entire, short, effuse at the base, 
angulated posteriorly; columella short, callous; the peri- 
stome entire, the left lip broad and thick, the right broadly 
margined. 
Fossil, Hocene, 6 species; Paris basin. Belgium, England, 
Punjaub. Cretaceous, 5 species; India, Gosau. 
PTEROSTOMA, Deshayes, 1864. 
Type, P. tuba. Eocene. Grignon, Paris. 
Shell elongated, turriculated ; peristome continuous, circular, 
very dilated and margined; columella very broad, expanded, 
and continuous with the peristome. 
ScALIOLA, Adams, 1860. 
Type, S. bella. 
Animal with the rostrum elongated, cylindrical, annulated, 
bifid at the end; tentacles filiform, eyes rrominent, black at 
the outer base of the tentacles; foot short, ovate, posteriorly 
sub-acuminate; operculum corneous, ovate, subspiral; nucleus 
subterminal, 
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