35fi LITTORINID.E. 



R. labiosa, Montagu. 



-, Brit. Moll. iii. p. 109, pi. 76. f. 5 ; pi. 77- f. 1, 2, 3 ; pi. 81 . 

 f. 3. 



R. Sarsii, Brit. Moll. iii. p. 101. 

 R. discrepans, Brown, Brit. Moll. iii. 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 point is blunt, from 

 whence a small upper lobe or alated membrane springs, on 

 which is fixed a suboval corneous operculum with spiral loose 

 stria?, 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 in com- 

 pany in the lower levels of the littoral zone, but are more 

 plentiful at the borders of the laminarian district. 



The R. labiosa is an aberrant form of this species. It has, 

 like the type, a smooth variety, and the gradations between 

 the type and synonyms can be well traced. The R. Sarsii and 



