374 



shell of three volutions of the same colour. Mantle simple. 

 Head a produced wrinkled muzzle, with a vertically cloven 

 yellowish- white disk, armed with the usual jaws, corneous 

 plates and tongue. The tentacula are short, flattish, blunt, 

 hyaline-white, not setose; eyes large, black, on mere pro- 

 minences at the external bases. Foot long, rather thick, 

 though narrow, slightly auricled, superficially labiated, and 

 tapering to a moderately elongated rounded termination. 

 The operculigerous lobe is much extended, well developed, 

 narrow anteally, dilated in a subrotund form posteally, tinged 

 with dark smoke-colour at the sides, edges, and on the upper 

 surface ; its extremity is of much the same shape as the main 

 foot or sole ; it does not terminate in a distinct cirrhus ; 

 indeed, the trace of one is obsolete. On the centre of this 

 upper lobe, considerably distant from the end, is fixed a 

 suborbicular operculum, that has much the same character of 

 the rapidly-increasing paucispiral turns as most of the Rissoa ; 

 the under surface of the foot is flake-white ; the upper part 

 and neck are marked with irregular, transverse, very fine, 

 close-set, pale lead-coloured lines. The animal, therefore, is 

 a simple Rissoid of discoidal figure, showing less deviation 

 from the type, R. parva, than most of the Rissoae with each 

 other. I feel pleasure in having the sanction of Professor 

 Forbes to this determination, who observes, in the ' British 

 Mollusca/ vol. iii. p. 156, "the Skeneae may be said to be 

 discoid Rissoa." 



The animal exhibits the organs freely, as I have remarked 

 is the case with all the littoral species, probably from the 

 frequent exposure of the branchial plume to the influence of 

 free air. This little creature marches rapidly, and is plentiful 

 on the finer algae of the pools, at Exmouth, in the half-tidal 

 levels. 



It has been objected, that the shell does not show the varix 

 at the external lip of the aperture, as in many of the Rissoce. 

 This objection is of no value, as many accredited Rissoae have 

 not the callus ; amongst them, jR. fulgida, R. soluta, R. vitrea, 

 R. proximo,, and others. And as to the foot being rounded 

 at its posteal extremity, that form is not uncommon in this 



