of the British Rissose. 255 



snow-white ; in progi'ession they are extended considerably be- 

 yond the head : the eyes are conspicuously black, and placed on 

 minute scarcely projecting external 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 ex- 

 tends to the limit of the body volution, and terminates poste- 

 riorly in a moderately pointed lanceolate shape; there is very 

 slight trace of a longitudinal line in the centre of the sole ; the 

 operculigerous lobe has the margins laterally so lax and dis- 

 united 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 oper- 

 culum of three spires, the two first minute and scarcely trace- 

 able; 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. 



Rissoa semistriata, Montagu. 



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. ulvce, in the 'Annals,' 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 

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

 contracted in the middle, pointed behind, and sometimes emar- 

 ginate, 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 temninates 

 behind with three caudal processes, whereof the middle is the 

 longest, and writing of it to Professor Forbes I termed it a 

 bashaw of three tails ; it carries the light corneous suboval oper- 



