LAND REGIONS. 87 
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 conti- 
nents, but on the islands, till every little rock in the ocean has 
its peculiar inhabitants—the survivors, seemingly, cf tribes 
which the sea has swallowed up. (Waterhouse.) ! 
The two largest genera, or principal types of the land and 
fresh-water shells, Helix and Unio, 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 similaris 
Fér. to Brazil and India; and often correctly, but only because 
they have been carried to distant localities by human agency. 
Land-snails are in fayour with Portuguese sailors, as ‘‘ live sea 
stock ;”” and they have naturalised the common garden-snail of 
Europe (Helix aspersa) in Algeria, the Azores, and Brazil; and: 
Helix luctea at Teneriffe and Mte. Video. <Achatina fulica has 
been taken from Africa to the Mauritius, and thence to Calcutta, 
where it has been established by a living naturalist; and Helix 
hortensis has been carried from the old country to America, and 
naturalised on the coast of New England and the banks of the 
St. Lawrence. Dulimus Goodall’, indigenous to the West Indies 
and S. America, has been intreduced into English pineries and 
to Mauritius. Helix pulchella, one of the small species found in 
moss and decayed leaves, inhabits Europe, the Caucasus, 
Madeira, the Cape (introduced), and N. America as far as the 
Missouri. Helix cellaria inhabits Europe and the Northern 
States of America, and has been carried abroad with the roots 
of plants, or attached to water-casks, and naturalised at the 
Cape and New Zealand. Testacella maugei has been transported 
from the Canary Islands to England. 
The fresh-water Pulmonifera—Limnea, Physa, Planorbis, 
Ancylus—and the amphibious Succinea, have a nearly world- 
wide range ; and ike aquatic plants end insects, often re-aprear, 
eyen at the antipodes, under familiar forms. The range of the 
gill-breathing fresh-water shells is more restricted. 
The Old World and America may be regarded as provinces of 
paramount importance, haying no species in common (except a 
* Tn cataloguing Unionidae, 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, ; whilst the latinised names 
of places often defy all attempts at re-translation. 
