of the British Bissose. 261 



scribed ; I therefore am inclined to consider it of that nature 

 which may be observed in the RissocE, when in creeping they 

 arrive at the level of the water, and commence exserting and 

 retracting with rapidity the buccal apparatus. 



In the animal just discovered the curious tails of the foot were 

 well developed, the angle of separation being about that of the 

 fore aud middle finger of the hand when placed as far apart as 

 possible. In all the animals I have observed I never met with 

 a similar termination of the main foot. 



Rissoa vitrea, Montagu. 



The animal occupies a pale yellow essentially smooth shell of 

 4^ tumid, though less rounded and more taper volutions, with 

 shallower sutures than the R. proxima. The general colour 

 of the animal is very pale dirty white. The mantle does not 

 protrude beyond the aperture. The head or rostrum is subcy- 

 liudrical, double the length of the 'proxima' invested with a 

 tunic to near its extremity, grooved above, emarginate at the 

 end, forming two minute, flat, symmetrical, arcuated, terminal 

 lobes, vertically cloven beneath as in jR. parva ; the colour on 

 both surfaces is a moderately suffused pink, through which the 

 buccal apparatus is visible ; and when the neck is much exserted 

 it will appear coloured with pale pink hues. The tentacula are 

 flat, much longer than in the * proxima,' but not clavate like it 

 at the tips ; they have however the same fine sharp setse at the 

 extremities ; the eyes are at the centre of the bases of the ten- 

 tacula, not raised, nor half the size of those of its congener. 

 The foot is truncate in front, very slightly labiated, with un- 

 usually short obtuse auricles ; not bifurcated posteriorly, but has 

 an entire somewhat taper and rounded termination not extend- 

 ing beyond the second volution : the operculum is fixed on a 

 simple lobe scarcely distinguishable from the upper part of the 

 foot ; it is rather more circular than in the last species, but, 

 though paucispiral, the turns are less distinct, the oblique striae 

 of increment coarser, and the colour instead of being clear white 

 is a dull yellow. All this is different in the 'proxima' 



The examples now described are the only two that have occurred 

 of this rare animal ; it and the 'proxima' were placed in the same 

 glass, and being lively I had good opportunities for comparison ; 

 the animals are organically different, but I think the 'proxima' 

 is a greater departure from the rissoidean type than the ' vitrea ' ; 

 still it will probably remain with the Rissoce, though some of the 

 specialties are on the verge of generic deviation. In the remarks 

 on these species I fear that conciseness is neglected, but the 

 confusion in which they have long been enveloped must be the 



