RISSOA. 



corneous jaws and buccal apparatus. The mantle is plain and 

 even. The tentacula are long, flat, not filiform, rather thick 

 at the base, tapering gradually to a rounded extremity ; they 

 are not setose : the large eyes are fixed on prominences at the 

 external angles. The foot at rest is short, on the march it 

 extends to the middle of the antepenultimate volution ; it is 

 labiated in front, but not auricled, constricted above instead 

 of in the middle, as is more usual in Rissoa, and then expands 

 and tapers to a narrowish attenuated rounded termination. 

 The operculigerous lobe dilates into subcircular lateral wings, 

 bearing close at the junction of the foot with the body, a sub- 

 oval, corneous, faintly spiral operculum, with the turns rapidly 

 increasing, as in the paucispiral Littorinae and typical Rissotc. 

 It has a distinct caudal cirrhus. 



Malacologists, from the curious sculpture and entire, flat, 

 striated, broad margin of the peristome of the shell, have 

 thought that this hitherto unrecorded animal would display 

 singular features ; but that is not the case ; it is a very simple 

 creature, and scarcely differs from the R. parva, except in 

 having the tips of the tentacula rather flatter, more rounded, 

 and in the different position of the constriction of the foot. 

 The animal is active, marches up a glass with uncommon 

 rapidity, and displays a freedom beyond the usual habits of 

 the tribe. It is found in all the zones. 



Some live examples of this species having recently occur- 

 red, I add to the above account, that the front part of the foot 

 is marked with an intense snow-white flake of the figure of the 

 letter V, visible in consequence of its transparency above and 

 below. I have also to remark, that the anterior terminal line 

 of the foot is unusually deeply incised, so as to form two labia ; 

 the lower, or that of the sole, at the centre part, on the march 

 is produced much beyond the upper lip. I had never before 

 seen this feature so extensively developed in any Rissoa. 



And lastly, I may state, that I failed to detect satisfactorily 

 the small pendant process in the mantle at the aperture, which 

 is so conspicuous in R. parva and R. semistriata yet it may 

 exist : I had the same difficulty in R. striata, but I saw it 

 afterwards in several examples. 



