RISSOA. 367 



rounded, deeply separated, rather oblique volutions, which are, 

 particularly the body one, closely, but superficially, spirally 

 striated ; the caducity of the striae renders this species very 

 liable to become glabrous from attrition. The general colour 

 of the external organs is a brilliant subhyaline white, but, 

 though aspersed with minute opake snow-flakes, the transpa- 

 rency is scarcely impaired. The mantle is even, and does not 

 emit a process from the portion that lines the upper angle 

 of the aperture. The head, when quiescent, is a short sub- 

 cylindrical rostrum, quite smooth and rounded at the termi- 

 nation ; 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 alimentary 

 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 pel- 

 lucid as to allow the intenser white canal or oesophagus lead- 

 ing 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 ex- 

 panding 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 man- 

 oeuvre is part of the animal oeconomy, or of the nature of that 

 which is sometimes seen in the typical Rissoa when disturbed, 

 I cannot determine until more specimens are observed; at 

 present, I think the action peculiar to this species. 



The tentacula are flat, strong, rather short, flake-white, 

 smooth, gently attenuating and becoming minutely clavifonn 

 at the tips, which are each clothed with six comparatively 

 long, intensely aciculate setje ; the eyes are unusually large, 

 black, and fixed on minute sub-semicircular lateral excres- 

 cences at the external bases, and are so amalgamated with 

 them as scarcely to present a prominence. The foot is a 

 curious organ, being large, fleshy, anteriorly grooved, so as to 

 form a slight labium, deeply indented in the centre, and pro- 



