BISSOA. 361 



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-pointed 

 lanceolate shape ; there is a very slight trace of a longitudinal 

 line in 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 striae 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 think 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. SEMISTRIATA, Montagu. 

 R. semistriata, Brit. Moll. iii. p. 117, pi. 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. ulvte, in 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 



