358 Mr. W. Clark on the Littorinid^. 



moderately long deeply-cloven annulate muzzle. Eyes at the 

 extremities of pedicles soldered to the shortish blunt tentacula, 

 being of concurrent length with them ; a canaliferous groove 

 runs from their bases to the branchial cavity. Foot large, broad, 

 auricled, truncate in front, with an obtuse posterior termination, 

 double-lobed ; the upper one, being much the smaller, carries the 

 usual horny suboval spiral operculum of the Littorinae. It in- 

 habits in sufficient abundance the small streams which discharge 

 into the Greenwich marshes, but generally within the reach of 

 the tidal and brackish waters. 



Rissoa ulvce. 



R. ulvce y '\ ^1 



R. suhumbilicata 1 J ^' 



Animal spiral, varying in colour from locality from nearly 

 black to pale brow*n ; mantle plain ; the head is a long dark pro- 

 boscidiform muzzle, emdrghiate in the centre in front, marked 

 with two transverse bars, and its margins edged with the same 

 dark colour ; mouth pale brown or yellow with a vertical fissure ; 

 tentacula very long, cylindrical, slender, pointed, of a frosted or 

 setose whitish yellow, wdth a black bar at a short distance from 

 their termini ; eyes at the external bases on short thick offsets ; 

 the foot is short, truncate and auricled in front, rounded, and 

 slightly, in some individuals, emarginate posteriorly. In the 

 lighter colour variety the upper part is a pale brown, and beneath 

 yellowish white with a border of minute pale golden flakes, with 

 sometimes a dark bar running through the centre of its posterior 

 part. In the darker variety the upper part of the foot is clouded 

 with very fine dark lead- colour to its paler hvies, underneath pale 

 brown. It carries a light suboval corneous operculum. These 

 animals creep with great rapidity, and float with the foot upper- 

 most by means of an hydrostatic apparatus, as air- bubbles are 

 seen continually to proceed from the aperture ; they are strictly 

 littoral, and inhabit in myriads the green oozes of the estuaries. 



I have no hesitation in consigning this species to the genus 

 Rissoa. It differs in many points from the true Littorince, espe- 

 cially in having the foot short, truncate, auricled in front, and 

 rounding to an obtuse point behind; its under surface is not 

 subdivided as in i. littorca ; it has the entire aspect of the foot 

 of the RissocB, except that it has not the posterior filamentary 

 appendage ; nevertheless there are the rudiments of it in the pre- 

 sent species ; and in its progression it has not a trace of the oscil- 

 latory action so conspicuous in the foot of the typical LittorirKs, 

 which I believe never swim, but the Rissoa in general are oftener 

 seen floating in a reversed position than otherwise. 



The shells of this section of the Rissoa are subject to great 



