72 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



belonging to the order Prosobranchiata and frequently known a* 

 Operculata, although some of these have no opercula. These 

 • comprise the Cydoplioridw, Cifclostomatidce, Acicidida', and perhapa 

 Truncatellidce belonging, like the freshwater Faludinidce, Valvatidce 

 and Ampullar idee, and many of the commonest marine univalves, to- 

 the Taenioglossate division of the suborder Pectinibranchiata, and the- 

 Helicinidce and JTydrocoenidce allied to Neritidce and appertaining to- 

 the Ehipidoglossate division of the suborder Scutibranchiata *. In. 

 the immediate neighbourhood of the Helicinidce there is another 

 terrestrial famil}^, the Proserpinidce^ without any operculum. The 

 great bulk of all the divisions and orders of the Prosobranchiata, it 

 is scarcely necessary to say, are marine. 



There is nothing in the present distribution of the various families- 

 of operculated land-shells antagonistic to the idea of each family 

 having originated from a distinct birthplace at a different period from 

 any other family. The Cydoplioridce, though found in all the prin- 

 cipal regions of the earth, are chiefly developed in the Oriental region,, 

 and are very largely continental in their distribution. The Cyclo- 

 stomatidce are best developed on what Wallace has termed ancient 

 continental islands, which have been separated from continent* 

 during the later Tertiary periods, and especially on the Antilles and 

 Mascarene Islands ; this family is represented only on the skirts of 

 the Oriental region. The Helicinidce, also mainly insular, have a 

 curiously different range from the Cyclostomatidce, although both are 

 chiefly developed in the West Indies ; Helicina extending through 

 the islands of the Pacific to Australia, the Malay Archipelago and 

 even Burma, but not to India. The last western straggler is found 

 in the Seychelles, and the family is unknown in Madagascar or in 

 Africa. The Proserpinidce are confined to Central America and the 

 West Indies. The last, judging by the small amount of differentia- 

 tion and dispersal that they have undergone, may be of more recent 

 origin than the others, whilst the abundance of the Cychstoma- 

 tldce on old continental islands and their poor development on 

 continents may show them to be of an older and less improved type 

 than the Cyclophoridce. Fossil forms of several kinds have been 

 found in Europe and elsewhere, but their affinities and even the 

 families to which they should be assigned are in general extremely 

 doubtful. 



Now this fact, that different subdivisions of an order, class or 

 fiubkingdom have in all probability originated at different periods in^ 



* The terms are those of Fisclier's ' Manuel de Conchyliologie.' 



