356 LITTORINID#®. 
R. labiosa, Montagu. 
—————— ;» Brit. Moll. ni. p. 109; pl. 76.4.5; pl. 77. £ 1,253 pls. 
f. 3. 
R. Sarsii, Brit. Moll. i. p. 101. 
R. discrepans, Brown, Brit. Moll. ii. p. 101. 
Animal spiral; mantle plain, just even with the shell. The 
head is a short dark brown muzzle, cloven with a vertical ori- 
fice beneath, the disk of which is yellow; tentacula long, 
slender and cylindrical, yellow, with a longitudinal row of 
white detached flakes, and often the reverse, white with 
yellow interrupted flakes ; eyes on short offsets at the external 
bases. Foot flaky-white above and beneath, long, narrow, 
truncate in front, slightly auricled; the poimt is blunt, from 
whence a small upper lobe or alated membrane springs, on 
which is fixed a suboval corneous operculum with spiral loose 
strie, and at its extremity a single short white tentacular 
filament is seen. The branchial plume consists of 15-18 
minute vessels attached under and to the mantle and back of 
the neck. 
This is one of the forms that conchologists have taken 
possession of for the manufacture of varieties into species. 
The type Rissoa parva is a most variable species, resulting 
from locality and other causes. The animals of all the 
varieties enumerated above are identical. We have examined 
them all, and can find no appreciable difference except va- 
rieties of colour and occasional variations in the length of the 
terminal filament, which is never in any two selected shells 
precisely similar. The R. costulata is an elongated variety ; 
the R. rufilabris a short tumid one, with a red or pink peri- 
stome, which may often be seen in its congeners. The R. in- 
terrupta is a more slender, thin, less plicated, dwarf variety. 
And lastly, the typical Rissoa parva varies so much that it is 
difficult to find two shells alike. These varieties live im com- 
pany in the lower levels of the littoral zone, but are more 
plentiful at the borders of the laminarian district. 
The A. dabiosa is an aberrant form of this species. It has, 
hike the type, a smooth variety, and the gradations between 
the type and synonyms can be well traced. The R. Sarsii and 
