362 LITTORINID^E. 



the external bases. The foot is nearly the shape of the type, 

 contracted in the middle, pointed behind, and sometimes 

 emarginate, but it is proportionately longer, larger, and 

 thicker ; there is no groove or longitudinal line on the sole ; 

 the upper lobe anteally expands into narrow white wings, and 

 terminates behind with three caudal processes, whereof the 

 middle is the longest, and writing of it to Professor Forbes, I 

 termed it a " bashaw with three tails" ; it carries the light 

 corneous suboval operculum at some distance from the end of 

 the foot, but the first two turns of the spire are nearly obso- 

 lete ; the third occupies the greater portion of the plate, and 

 is well marked with oblique lines of growth. 



I have lately examined many lively specimens, and can 

 confirm the fact of the operculigerous lobe terminating in 

 three filaments, as well as the presence of the short mantellar 

 process, that is produced and retracted at the will of the animal, 

 from the upper angle of the aperture. What are the func- 

 tions of this organ is doubtful ; it has not the aspect, nor the 

 position, of a reproductive element ; it has more the resem- 

 blance of a tentacular instrument; but in some Rissoce it 

 acquires an imperfect tubular appearance, as in the Chem- 

 nitzia, in which, particularly Ch. acuta, it seems to perform 

 the office of the branchial siphon of the Canalifera. I believe 

 that this appendage has scarcely been noticed by authors ; it 

 appears to exist in many of the Rissoa, but if in all is doubt- 

 ful ; it has no connection with the operculigerous lobe, or its 

 wings or caudal cirrhi, but is a strictly mautellar process. 

 The animal is free, unusually rapid on the march, inhabits all 

 the zones, and has not been before observed. 



R. COSTATA, Adams. 



R. costata, Brit. Moll. iii. p. 92, pi. 78. f. 6, 7, and iv. p. 263. 

 Animal inhabiting an elaborately sculptured, costated, spi- 

 rally striated, basally ridged, pale yellow shell, of 5-6 rounded 

 volutions, hyaline-white, except the large black eyes and pale 

 red biiccal disk. Head a long proboscidiform muzzle, finely 

 corrugated in quietude, cloven vertically at the orifice, as in 

 R. purva, but showing more partially than in that species the 



