204 Mr. Cosmo Melvill on the 



(XI.) Groups of tzuo or three in close affinity with each- 

 other, otherzvise soniezvhat isolated. 



C. talpa (L.). 



C. exits ta (Sowb.). 



Both these must surely have descended from a common 

 ancestor, but the distinction is never failing, and known at 

 once. The latter differs from the commoner and more 

 widely distributed talpa in being more pyriform in shape, 

 teeth more numerous, smaller, and somewhat immersed,.- 

 aperture narrower. While G. talpa is found commonly in 

 the Eastern tropics, C. exusta is confined to the Red Sea,, 

 but it is a great deal more than a local variety. 



C. inns (L.) 



C. leucostonia (Gaskoin). 



The latter, which is rarer, has often been confounded 

 with C. niiis^ indeed, it was not till 1843 that Mr. Gaskoin 

 differentiated the two. The complete absence of teeth 

 in the latter, the white base, and the heavier and white 

 appearance of the shell, with one or two other distinctions 

 of minor importance, amply distinguish them : but they are 

 not nearly allied to any other of the genus, though a fossil 

 form or two is rightly connected with them by M. F. 

 Jousseaume, Natives, C. mus of the Mediterranean, and 

 W. African shores., C. leucostonia, Arabian coasts. 



C. lurida (L.). 



C. pidchra (L). 



C. controversa (Gray). Isabella (L.) var. 



On the upper side these three are very similar, all being 

 bluish-cinereous with two brown spots at either extremity. 

 Below, however, the common lurida is wide-mouthed, with 

 coarse dentition, C. pulchra and controversa have a narrower 

 aperture, but the latter is pure white both as to the base and 

 teeth, with the extremities yellow spotted as in its type C. 

 Isabella, the former being olivaceous, with small, fine, some- 

 what obsolete teeth, of a reddish tint. 



