1868.] SPECIES OF MELO AND CYMBA. 375 



distinguishing them, viz. the greater immersion of the spire in M. 

 ducalis, I find to be quite fallacious. So variable, in fact, is the 

 more or less immersed state of the spire in what would be consi- 

 dered well-marked specimens of both forms, that I cannot regard it 

 in anj' other light than a purely individual peculiaritj'. 



The difference of locality may doubtless be urged, in the case of 

 JLf. diadema, as a reason for keeping it distinct ; but although a 

 difference of locality, when joined with other marked peculiarities, is 

 no doubt a strong additional proof of sjiecific distinctness, I do not 

 think that it of itself, in the absence of such other peculiarities, 

 tends to prove the specific value of any particular form. 



Cymba porcina, Lamarck. 



This species has long been a bone of contention amongst concho- 

 logists, many taking the view that it is merely a variety of C. 

 proboscidalis, Lamarck, others as stoutly maintaining its specific 

 distinctness. Reeve admits its specific difference, but with some 

 hesitation. 



Having examined between twenty and thirty examples of the two 

 species, ranging in length from 1|- inch to 8 or 9 inches, all from 

 the same locality, I must say I have failed in making out a specific 

 difference in the two forms. 



The number of the columellar plaits, which is a character much 

 relied on for distinguishing between the two forms, is not, I think, 

 by any means an infallible guide. I have seen an undoubted speci- 

 men of the proboscidalis form with only two columellar plaits, and 

 others in which the fourth plait was scarcely appreciable. All the 

 specimens of C. porcina I have hitherto met with had three colu- 

 mellar plaits. 



Cymba navicula, Gmelin. 



I cannot distinguish specimens of this shell from some young 

 forms of C. neptuni, which are always mottled with white, except 

 by the number of the columellar plaits. The difference of number 

 in these jilaits, which, as mentioned above, is occasionally a variable 

 character, appears in this instance, however, to be constant ; it may 

 therefore perhaps be better, taking this into account with other 

 slight peculiarities, to keep them distinct. 



Cymba patula, Broderip. 



Is this the young of C, neptuni, or a distinct species ? asks i\Ir, 

 lleeve. From the specimens I have examined I am inclined to 

 think that it is certainly the latter. It appears to me to be more 

 nearly allied to C. oUa, young specimens of which it closely resem- 

 bles, than to C. nejytnyii. 



