358 Mr. W. Clark on the Littorinide. 
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 Littorine. 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 ulve. 
R. ulve, | é 
R. subumbilicata r} Montagu. 
Animal spiral, varying iv colour from locality from nearly 
black to pale brown ; mantle plain ; the head is a long dark pro- 
boscidiform muzzle, emarginate 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, poimted, of a frosted or 
setose whitish yellow, with 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 g 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 tu its paler hues, 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 Littorme, espe- 
cially in having the foot short, truncate, auricled in front, and 
rounding to an obtuse poimt behind; its under surface is not 
subdivided as in ZL. littorea ; it has the entire aspect of the foot 
of the Rissoz, 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 Lettorine, 
which I believe never swim, but the Rissoe in general are oftener 
seen floating 15 a reversed position than otherw wise. 
The shells of this section of the Rissoe are subject to great 
