RISSOA. 363 
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 Littorine and typical Rissoe. 
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 im 
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. 
