376 LITTORINID ZL. 
suffused with lemon-yellow, and studded with distinct flake- 
white points; there is very little trace of a depressed medial 
line on the sole. 
A subcircular, paucispiral, corneous operculum, with a sub- 
central nucleus, is fixed on an alated, very elongated upper 
lobe, narrow anteally, dilating behind, marked with minute 
smoke-coloured blotches at the sides and edges, as in Rissoa, 
and with more intensity on the upper surface. The operculum 
is placed on a circular dilatation at the centre of the lobe, 
which is produced considerably beyond it to a blunt, though 
lanceolate-shaped point, which laps on the upper surface of 
the posterior part of the foot, at some distance from its termi- 
nation, and appears to be the locum tenens of the typical 
Rissoid cirrhus, though it is not what is understood as strictly 
cirrhal. These sort of pedal filamentary appendages are of 
no generic value, and from the uncertainty of their presence 
in many of the Rissoe, of scarcely specific importance. 
This species is perhaps the commonest that exists; it is 
most abundant on the lower littoral levels; the animal is 
omnivorous, feeding on animal matters, and on Alge and 
Corallina officinalis ; it is equally plentiful in the deepest sea 
districts ; its variations in figure are endless; every locality 
has its variety. The animal is as free as the typical Rissoe 
in showing its organs. . 
These notes show that this so-called Cerithium, in organic 
structure, is a perfect Rissoa. Even the shell, as in that genus, 
has the characteristic varix at the outer lip, and in the pauses 
of growth, similar varices declare that the aperture is always, 
when the animal is not on the increase, provided with the 
typical callous pad. 
We conclude by observing, that if the R. parva be consi- 
dered the type of the genus, and it be not at hand, the 
R. planorbis, or this species, malacologically, has greater and 
more appropriate claims to act as a vice-type than any other 
Rissoa in the list. We cannot comprehend how three genera 
can with propriety be constituted from essentially identical 
animals. 
We think that the Murex adversus of Montagu—a Ceri- 
