RISSOA. 36] 
offsets. The foot is truncate in front, grooved so as to form a 
labium, and slightly auricled; it is altogether stronger than 
in R. parva, and like it rarely extends to the limit of the body 
volution, and terminates posteriorly in a moderately-poimted 
lanceolate shape; there is a very slight trace of a longitudinal 
line mm the centre of the sole. The operculigerous lobe has 
the margins laterally so lax and disunited as to form wings, 
which on the march, at the will of the animal, continually 
change their appearance ; on it, near the junction of the foot 
with the body, is the light horny oval operculum of three 
spires, the first two minute and scarcely traceable; the last 
enlarges rapidly, and shows distinct striz of growth. There 
is no caudal cirrhus on the opercular lobe, which is nearly 
coextensive with the main foot, and so pointed as to appear 
like one, and in some specimens is unequally emarginate at 
the sides. 
The animal is not shy, and marches with vivacity ; it has 
been remarked that it is disproportionately small for the shell, 
and the tentacula very short; I do not thmk our southern 
examples confirm this view: it inhabits all the districts. The 
records of this common species are so scanty, that it may 
almost be looked on as undescribed. 
R. semistri1ata, Montagu. 
R. semistriata, Brit. Moll. iu. p. 117, pl. 80. f. 4, 7. 
The animal occupies a shell of six moderately convex volu- 
tions, each partially striated, the middle portions being 
smooth. Its colour is nearly white, with the palest tinge of 
yellow. The mantle is even with the shell, except a short 
minute filament that is protruded by the animal from the 
portion which lines the upper angle of the aperture, like that 
I have mentioned in the Rissoa parva above, and in the 
R. ulve, m the ‘Annals of Natural History, N.S. vol. vi. 
p- 33. The head is a short muzzle, not so long as in the 
type, but similarly grooved in the centre above, and cloven at 
the extremity and below. The tentacula are flattish, rather 
long, divergent, frosted, pale yellow or white, with the tips 
slightly clavate; the eyes are on gently raised prominences at 
