of the British Rissose. 259 



tions. The mantle does not extend beyond the aperture. The 

 colour is hyaline white with a trifling exception. Head probos- 

 cidiform, having its terminal pale red disk vertically cloven, in 

 which the buccal organs are distinctly visible. The tentacula 

 are moderately long, rounded at the tips, very pilose, the setse 

 springing from them horizontally, but only visible with high 

 powers j eyes at the external angles on small scarcely raised pale 

 sulphur-coloured eminences. Foot subtruncate, slightly auricled, 

 labiated or grooved anteally, and long and narrow. Operculi- 

 gerous lobe small and not much alated ; no caudal cirrhus was 

 detected ; the light corneous operculum is suborbicular and pau- 

 cispiral, fixed nearly at the extremity of the foot. The animal 

 is exceedingly vivacious and free, marching up a glass with sin- 

 gular rapidity. It is very abundant alive in the coralhne zone, 

 in 14 fathoms water, off Budleigh Salterton, Devon. 



Rissoa proximttj Brit. Moll. 



The animal inhabits a thin sordid white shell of four rounded, 

 deeply separated, rather oblique volutions, which are, particularly 

 the body one, closely but superficially spirally striated; the 

 caducity of the strise renders this species very liable to become 

 glabrous from attrition. The general colour of the external or- 

 gans is a brilliant subhyaline white, but, though aspersed with 

 minute opake snow flakes, the transparency is scarcely impaired. 

 The mantle is even, and does not emit a process from the por- 

 tion that lines the upper angle of the aperture. The head when 

 quiescent is a short subcylindrical rostrum, quite smooth and 

 rounded at the termination ; it is not tunicated, lobed, grooved, 

 nor vertically cloven on the upper part, and on the lower area it 

 forms a disk, which has not a distinct vertical fissure as in 

 R. parva, but shows a fine crosial incision, which is the ali- 

 mentary orifice, and within it are probably the corneous jaws 

 and buccal apparatus, but I could not detect them, perhaps from 

 being of the hyaline colour of the rostrum, which is so pellucid 

 as to allow the intenser white canal or oesophagus leading to the 

 stomach to be seen through the walls. 



When the animal is on the march it often suddenly evolves 

 the rostrum to double its usual length, at the same time expand- 

 ing the termination into a large disk or finely dentated flattened 

 rose, which it throws back on the margin of the upper point of 

 the aperture, and then as quickly withdraws the extension to its 

 usual limits ; whether this curious manoeuvre is part of the ani- 

 mal oeconomy, or of the nature of that which is .sometimes seen 

 in the typical Rissoce when disturbed, I cannot determine until 

 more specimens are observed; at present, I think the action 

 peculiar to this species. 



17* 



