34 Mr. W. Clark on the Lacunae. 



parva in the May * Annals/ 1850, page 359, by mistake, may be 

 considered as cancelled, as well as the parts of lines 39 and 40 

 now quoted : " The R. scalariformis has the plicse more numerous, 

 white and delicate/' 



I am. Gentlemen, your most obedient servant, 



William Clark. 



Exmouth, June 17th, 1850. 



Postscript. — Since the above was in type, I have reviewed 

 at this place, in a living state, all the Lacunce of authors, except 

 the " crassior," and I think I am authorized to be still more 

 " tenax propositi,^' that they are a section of the genus Littorina, 

 and cannot be severed from it without violence to true generic 

 value. I find them all with variable excrescences of the opercu- 

 ligerous 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 

 " quadrifaaciata,'' a variety of the " vincta," besides the acces- 

 sories of the "pallidula,'" has a very short style at the central 

 posterior point of the pedal disc ; the " crassior " I have men- 

 tioned above. The Lacuna fasciata of authors is referable to the 

 L. vincta, and not to the L. puteolus. 



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 appen- 

 dage of the dog, even when unmutilated, are very variable ; con- 

 trast 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 molluscum or the dog, have generic value ; 

 if any of these animals were without caudal terminations and 

 operculigerous lobes, that circumstance might contribute to esta- 

 blish generic value ; but I speak with reserve on this point, be- 

 cause in Pleurotoma, malacologists do not separate the species 

 with operculigerous lobes from those which have none, as the 

 P. septangulare with a decided operculum, and P. Ginnanianum 

 without a trace of one, are styled Pleurotomata ; here the total 

 absence or presence of opercula has not hitherto been held of 

 importance in a generic point of view, a fortiori; mere variations 

 of these organs can be, in this respect, of little value. The points 

 of support, then, for the establishment of the genus Lacuna, rest 

 on the lateral and caudal variations of the operculigerous lobes 

 of the foot, some difference in the organe generateur and in the 

 lacuna or groove in the columella of the shell, which is merely 



