RISSOA. 355 
are very dark; it carries a light, corneous, suboval, pauci- 
spiral operculum, with fine oblique striz. The foot is per- 
fectly rounded behind, and in almost all specimens more or 
less emarginate, though in some scarcely visible, in others 
decidedly so, at several points of the arcuation ; it is truncate 
and well auricled in front, without a medial line. There is 
invariably a very short cylindrical process or fillet exserted 
from that part of the mantle which lines the anal canal or 
upper angle of the aperture: I am unable to say anything as 
to its use: whether such an appendage exists in other minute 
congeneric species, and from its minuteness has passed with- 
out observation, must be left for future examination. I con- 
sider this species a Rissoa. Though it has not the caudal 
filament and pointed pedal termination of the typical Rissoe, 
we must not forget that there are some Rissoe with rounded 
tails and without a caudal cirrhus at the termination of the 
operculigerous lobe. 
I have elsewhere said, that every British Rissoa must have 
a distinct genus, if we insist on a literal agreement with the 
type R. parva; they all differ as much, or even more, from 
it, than the R. ulve; indeed, this species only varies in the 
broader foot, its rounded posterior termination, and in being 
without a cirrhus. Some authors deposit the R. ulve, with 
one or two more species of the estuaries, in Pfeiffer’s Palu- 
dinella: surely this is sad generic refinement. Since the 
observation above, on the filament springing from that part 
of the mantle which lines the upper angle of the aperture, I 
can report, that it is present in almost all the Rissoe, but its 
precise use is not yet satisfactorily made out. 
R. parva, Da Costa. 
R. parva, Brit. Moll. iii. p. 98, pl. 76. f.2, 6; pl. 77. f.6,7; pl. 82. 
f. 1+4. 
R. interrupta, Brit. Moll. iii. p. 100; et Auct. 
R. costulata, Alder. 
————., Brit. Moll. iii. p. 103, pl. 77. f. 4, 5. 
R. rufilabris, Alder. 
, Brit. Moll. in. p. 106, pl. 77. f. 8, 9. 
