LITTORINA. 351 
them all with variable excrescences of the operculigerous lobe 
of the foot. The Lacuna puteolus, which is a distinct species, 
has the lateral appendages large, but the caudal filaments very 
short, and often obsolete: the L. pallidula has the lateral wing- 
like extensions, with 2—4 very short caudal fillets: the “ guadri- 
fasciata,” a variety of the “vincta,”’ besides the accessories of 
the “pallidula,” has avery short style at the central posterior 
point of the pedal disc: the “ crassior” I have mentioned 
above. The Lacuna fasciata of authors is referable to the 
L. vincta, and not to the L. puteolus. I may state that the 
caudal and operculigerous accessories are not only variable in 
the species, but the individuals of each are also subject to 
variations in these parts. 
It appears very doubtful if these alated lateral extensions 
and caudal filaments are of much generic value, from their 
variableness in many species, and almost total absence in 
others ; I think that they may be considered as analogous to 
the variation of certain organs in other animals. We may 
observe, that the caudal appendages of the dog, even when 
unmutilated, are very variable: contrast the greyhound with 
the setter, terrier, bull or shepherd’s dog. But surely no one 
will contend that the variations in these points, either in the 
mollusk or the dog, have generic value. If any of these ani- 
mals were without caudal termimations and operculigerous 
lobes, that circumstance might contribute to establish generic 
value. The points of support, then, for the establishment 
of the genus Lacuna, rest on the lateral and caudal varia- 
tions of the operculigerous lobes of the foot, some difference 
in the organe générateur, and in the lacuna or groove in 
the columella of the shell, which is merely an imcomplete 
umbilicus that is very conspicuous in some species, in others 
almost closed up; and often in the same species, the Puteoli, 
it is sometimes apparent, and frequently scarcely visible: in 
all other respects these Lacune are similar to the Littorine. 
In opposition to these pretensions, I repeat, that all the so- 
called Lacune have invariably the form of the pedal dise of 
the Littorine, accompanied by the overwhelming characteristic 
vermicular alternate quality of progression which is almost 
