374 MR.T. G. PONTON ON CERTAIN [May 28, 



7. A Review of some of the Species of the Genera Melo and 

 Cymba of Broderip. By Thomas Graham Ponton, F.Z.S. 



Mr. Reeve, in his monograph of these genera (which, while ac- 

 knowledging their undoubted generic value, he nevertheless unites 

 under the old name of Gymbium), describes seventeen species, nine 

 belonging to Melo, and eight to Cymba. 



Having lately had the opportunity of examining a number of 

 specimens of some of these, I propose to say a few words as to their 

 specific value. 



Mklo georgin^. Gray. 



This species was first described by Dr. Gray in the year 1833 ; 

 but in his subsequently published monograph of the Volutidcd he 

 considers that both it and M. ducalis (Lk.) are varieties of M. 

 diadema, Lk. Reeve, in his monograph of Cymhium, says he con- 

 siders this species to be undoubtedly distinct both from M. diadema 

 and M. ducalis, with which latter species he, however, unites M, 

 umhilicata, Broderip. 



After a very careful examination of a considerable number of 

 specimens of M. ducalis, M. umbilicata, M. georgince, and M. dia- 

 dema, I cannot help thinking that Reeve is right in uniting M. 

 umbilicata with M. ducalis ; but, as regards both that species and 

 M. georgince, that he is wrong in separating them from each other 

 and from M. diadema. 



I am quite unable to distinguish il/. georgince from M. ducalis \ 

 they appear to me to pass into each other and into M. diadema by 

 a series of gradations. 



First, with regard to M. georgince, it seems to me that the slight 

 differences in form and coloration which have been mainly relied on 

 for distinguishing them, when a number of specimens of both arc 

 brought together, entirely lose their value. The deep-red hue and 

 linear markings which have been supposed to characterize M. 

 georgince do not, I find, really do so. I have seen more than one 

 specimen oi II. ducalis which possessed the linear markings in com- 

 bination with the brilliant red-brown reticulations on the bright flesh- 

 coloured ground usually fovind in that form. The greater ventri- 

 cosity of form also of M. georgince I find to be very variable. Spe- 

 cimens of M. ducalis and M. georgince of equal length frequently 

 give equal measurements in breadth also. 



Another reason for uniting species presenting such luidoubted 

 intermediate forms is the similarit}' of habitat. For, to cite the 

 converse of a proposition recently laid down to define the limits of 

 a species, " When two different forms are found in the same habitat, 

 and are connected together by intermediate forms, such forms can- 

 not be considered specifically distinct." But, supposing that M. 

 georgince passes into M. ducalis, does that species merge into M. 

 diademal Most undoubtedly I think it does, and that by inde- 

 finable gradations. One character not unfrequently relied on for 



