176 Memoirs öf the Indian Museum. [Vol. VIL 



more elongate, on an average, in Dolium melanostoma than in Dolium variegatum, 

 though further information is necessary to make certain about this point. The 

 narrow intercalary ribs appear to alternate with the larger ones much more regularly 

 in Dolium melanostoma than in either Dolium galea or Dolium variegatum. In this 

 respect, as also with regard to the feebly sunken sutures, Dolium melanostoma recalls 

 Dolium chinense, Dillwyn, but its ribs do not become so completely flattened out as in 

 the last-named species. Lastly, Dolium melanostoma, like Dolium galea, lacks the 

 brown maculations of Dolium variegatum and Dolium chinense. 



We may conclude that Dolium melanostoma is specifically distinct both from Dolium 

 galea and from Dolium variegatum. There exists therefore, at the present day, a 

 group of three closely related species, each with its special geographical distribution : 

 Dolium galea inhabiting the Mediterranean and Atlantic, Dolium variegatum charac- 

 terising the Indian Ocean and western shores of the Pacific, and Dolium melanostoma 

 in the central Pacific region. As pointed out on a previous occasion, it is improbable 

 that so conspicuous a species as Dolium variegatum should have been overlooked in 

 the Bay of Bengal and Malay region if it really lived in those portions of the ocean ; 

 and its occurrence in the northern part of the Arabian Sea suggests therefore a 

 discontinuous distribution for that species. It is known to exist along the northern 

 and eastern coasts of Australia, but we cannot at present ascertain whether it spreads 

 continuously across the intervening seas from Australia to Japan, in which latter 

 region it is also known to occur, this being the habitat of Dolium japonicum, Dunker, 

 synonymous with Dolium luteostoma and with Dolium variegatum. We know that 

 Dolium variegatum has been in existence since miocène times, and from the distribu- 

 tion of the fossil occurrences, we may conclude that its present discontinuous distri- 

 bution is due to the local shrinking of a once connected area. No fossil occurrences of 

 Dolium galea or Dolium melanostoma have been discovered, but our knowledge of the 

 later tertiary marine faunas of tropical regions is as yet too incomplete to lay stress 



upon this circumstance. 



II. 



The second point to be discussed is the relationship to Dolium, chinense, Dillwyn, 

 of a form described in 1904 by G. B. Sowerby as a new species under the name of 

 Dolium magnificum (Proc. Malac. Soc. London, Vol. VI, p. 7, fig. 1). The specimen 

 described and figured by Sowerby was obtained by the late General Tripe from China, 

 from the very home, therefore, of Dolium chinense. 



We have evidently to deal, here, with another of the numerous instances in which, 

 in the genus Dolium, large individuals are apt to differ considerably in appearance from 

 smaller specimens of the same species ; though in the present case the difference is not 

 of an extremely marked degree. The specimen of Dolium chinense figured in Reeve's 

 monograph measures 75 mm. in height. A specimen of nearly the same size, closely 

 corresponding with the one figured by Reeve, is to be found in the collections of the 

 Indian Museum, together with some smaller ones and a much larger one which will be 

 further alluded to. The maximum height of Dolium chinense, according to Küster, is 

 3^", therefore more than 80 mm. The type-specimen of Dolium magnificum has a 



