COOKE: ON THE GENUS CUMA. 171 



quiformis, of which latter I have many specimens without the 

 symptom of a groove. 



The operculum is the same in both. 

 There remains the columella^ and here alone there appears any 

 distinction which really may be taken as constituting generic 

 difference. The 'convexity' of the columella in Cuma, as com- 

 pared with its ' flatness ' in Piirpicra, may be dismissed at once, 

 for in the so-called Cuvm carijiata, kiosquiformts, rugosa, sacellum, 

 the columella is just as straight as in the normal Purpura. 

 There remains only the sentence — ' sometimes with a strong 

 angular tooth in the middle.' If for 'sometimes' we read 

 ' once,' the facts will be stated correctly. For, out of the five 

 species to which we have reduced Cuma, tectum alone possesses 

 this ' strong angular tooth.' It follows, therefore, that the re- 

 maining four, destitute as they are of what we have proved is 

 the sole qualification for generic difference, and unable to pro- 

 duce any other claim to the same, must sink back into what 

 they doubtless ought to be, viz.. Purpuras pure and simple. 

 Cuma then, is represented, really on the showing of the genus- 

 makers themselves, by the single species tectum. 



It is hardly correct to describe this sole peculiarity of 

 Cuma as simply ' columella with a strong angular tooth in the 

 middle.' For, if a shell of Cuma, tectum be broken open, it 

 will be seen that this 'tooth' is not a development of the colu- 

 mella only, not a mere sharpened callus, so to call it, but is 

 the termination, on the columella, of a strong ridge which 

 ascends the interior of all the whorls, up to the top of the spire 

 itself. Hence its true importance ; for a mere callus, confined 

 to the columella, would not take equal rank as determining a 

 genus. 



Lastly, I would point out that the evidence of geographical 

 distribution is in favour of this restriction of the genus Cuma. If 

 the other four species, carinifera, gradata, kiosquifortnis, and 

 sacellum, which I hope I have succeeded in expelling from the 

 genus, are placed here, there is no explanation of the fact that a 



