OCR Text |
Show 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 T H O M A S G R A H A M P O N T O N , F.Z.S. Mr. Reeve, in his monograph of these genera (which, while acknowledging their undoubted generic value, he nevertheless unites under the old name of Cymbium), 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. MELO GEORGINAE, Gray. This species was first described by Dr. Gray in the year 1833 ; but in his subsequently published monograph of the Volulidee he considers that both it and M. ducalis (Lk.) are varieties of M. diadema, Lk. Reeve, in his monograph of Cymbium, says he considers this species to be undoubtedly distinct both from M. diadema and M. ducalis, with which latter species he, however, unites 31. umbilicata, Broderip. After a very careful examination of a considerable number of specimens of M. ducalis, M. umbilicata, M. georgince, and M. diadema, 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 M. georgince from M. ducalis; they appear to m e 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 are 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 of 31. ducalis which possessed the linear markings in combination with the brilliant red-brown reticulations on the bright flesh-coloured ground usually found in that form. The greater ventri-cosity of form also of 31. georgince I find to be very variable. Specimens of 31. ducalis and M. georgince of equal length frequently give equal measurements in breadth also. Another reason for uniting species presenting such undoubted intermediate forms is the similarity of habitat. For, to cite the converse of a proposition recently laid down to define the limits of a species, " W h e n two different, forms are found in the same habitat, and are connected together by intermediate forms, such forms cannot be considered specifically distinct." But, supposing that 31. georgince passes into M. ducalis, does that species merge into 31. diadema 1 Most undoubtedly I think it does, and that by indefinable gradations. One character not unfrequently relied on for |