188 LAND REGIONS. 



introduced and acclimated since tlie occupation of these islands 

 by the Spaniards and Portxiguese, as they are confined to the 

 cultivated portions of the islands and do not occur in the latest 

 fossil deposits. The genus Craspedoponia is characteristic of 

 this fsxuna. No species of Unio occur, and in the Azores, not a 

 single species of fluviatile mollusca. In Madeira the characteristic 

 groups of Helices are Leptaxis, Janulus, Actinella, Ochtephila, 

 Phlebecula, Tectula ; in the Canaries, Hemicyclus, Moniliaria, 

 etc. The ancient sandy dunes of Cani9al, at the eastern extremit}'^ 

 of Madeira and in Porto Santo, contain numerous terrestrial 

 mollusks, some of which have not been found in a living state. 

 As the fossil examples of several species are larger than their 

 living descendants, it is possible that some of these species 

 reputed to be extinct have only degenerated. It is a remarkable 

 fact that some of the comnaonest living species are not found 

 fossil, whilst others, now extremely scarce, occur abundantly as 

 fossils. Helix tiarella was supposed to be extinct until 1855, 

 when Mr. Wollaston detected it alive in two almost inaccessible 

 spots on the north coast of Madeira. The degeneration of the 

 laud shells in variety and size may be the result of a modern 

 change of physical conditions brought about by human agency. 

 The annual fall of rain is now 29.82 inches, whereas it was 

 remarked by Columbus, nearly 400 years ago, " that, formerly, 

 the quantit3^ of rain was as great in Madeira, the Canaries, and 

 the Azores, as in Jamaica, but since the trees which shaded the 

 ground had been cut down, rain had become much more rare." 

 In the Azores there are Yl species, in Madeira 164, in the 

 Canaries 189, Cape Verd 40; some of these are common to two 

 or more groups, and some of the 50 introduced species in each 

 case form part of the enumeration. 



T. Central African Region. 



This region, limited to the North, by the Desert of Sahara, 

 includes the whole of intertropical Africa, with the exception of 

 the basin of the Gulf of Guinea. Some of its species are 

 distributed northward, following the course of the Nile, as far 

 as LoAver Egypt. The characteristic genera are Limicolaria, 

 Achatina, Ennea, Lanistes, ^theria, Iridina and Spatha. There 

 are scarcely any Helices and the operculated land shells are very 

 few. The three first genera enumerated are bulimoid land shells, 

 Achatina including a number of very large species, only 

 comparable in size with the Bulimi of the Borus group inhab- 

 iting Brazil, etc. There are a few fresh-water pulmonates, the 

 Physse being represented by the groups Physopsis and Isidora ; 

 but most of the fluviatile nnivalves are gill-breathers, including 

 Melania, Paludina, Ampullaria, and the peculiarly African 

 group Lanistes ( reversed Ampullaria). There are some Unios, 



