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 " quadri- 

 fasciata" a variety of the " vincta,'' besides the accessories of 

 the " pallidula" has a very 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 

 umnutilated, 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 clog, have generic value. If any of these ani- 

 mals were without caudal terminations 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 organs yemrateur, and in the lacuna or groove in 

 the columella of the shell, which is merely an incomplete 

 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 Lacunas are similar to the Littorince. 

 In opposition to these pretensions, I repeat, that all the so- 

 called Lacunffi have invariably the form of the pedal disc of 

 the Littorinee, accompanied by the overwhelming characteristic 

 vermicular alternate quality of progression which is almost 



