TRANS-ATLANTIC PROVINCE. 3S1 



Donax variabilis. G. F. Periploma papyracea. M. Y. 



Solecurtus fragilis. M. SC. Lyousia hyalina. Y. 



„ caribbaeus. M. F. Pandora trilineata. M. F. 



Corbula contricta. M. F. Pholas costata SC. F. 

 Periploma Leana. M. Y. ,, semicostata. SC. 



LA.ND REGIONS. 



Distribution of Land and Fresh -water Shells. 



The boundaries of th.e Natural-history land-regions are more distinctly 

 marked, and have been more fully investigated, than their counterparts in the 

 sea. Almost every large island has its own fauna and flora ; almost every river- 

 system its peculiar fresh-water fish and shells; and mountain-chains like the 

 Andes appear to present impassable barriers to the " nations" of animals and 

 plants of cither side. Exceptions, however, occur which shew^ that beyond 

 this first generalisation there exists a higher law. The British Channel is 

 not a barrier between two provinces, nor is the Mediterranean ; and the desert 

 of Sahara separates only two portions of the same zoological i-egion. In 

 these and other similar instances the " barrier " i s of later date than the sur- 

 rounding fauna and flora. 



It has been often remarked that the northern part of the map of the 

 world presents the appearance of vastly-extended, continental plains, much of 

 which is, geologically speaking, nev«f land. In the southern hemisphere the 

 continents taper oft' into promontories and peninsulas, or have long since 

 broken up into islands. Connected with this is the remarkable fact that only 

 around the shores of the Arctic Sea are the same animals and plants fouiwU- 

 through every meridian ;''and^l;Slir'rn^p1issing southward, along the three 

 prlhcipanmes of land, specific identities give way to mere identity of genera, 

 these are replaced by family resemblances, and at last even the families of 

 Animals and plants become in great measure distinct — not only on the great 

 continents, but on the islands — till every little rock in the ocean has its 

 peculiar inhabitants — the smfvivors, seemingly, of tribes which the sea has 

 swallowed up . ( Waterhoase. ) 



The two largest genera, or principal types of the land and fresli- water 

 shells, Helix and JJnio, have an almost universal range, but admit of many 

 geographical subdivisions.* Amongst the land-snails are several species to 

 which a nearly world -wide range has been assigned, sometimes erroneously 

 as when Helix cicatricosa is attributed to Senegal and China, or Helix simi- 

 laris Eer. to Brazil and India ; and often correctly, but only because they 

 have been carried to distant localities by human agency. Land snails are in 



* In cataloguing Unionida the river and country of each species should be stated. 

 American authors are too often contented with recording such localities as " Nash- 

 ville " and " Smithville,"' which are quite unintelligible. Almost as uncertain in their 

 meaning are S. Vincent, S. Cruz, S. Thomas, Prince's Id. ; Mhilst the latinized names 

 cf places often defy all attempts at re-translation. 



