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Porto Santo^ and follow this Protean Helix into the 

 other divisions of the group ; we meet with it on the 

 Dezertas as the H. senilis (those moreover from the 

 central island having a much more open umbilicus than 

 is the case in the northern and southern ones)^ whilst in 

 Madeira proper it constitutes the H. lincta (with an 

 additional pale variety for the calcareous district of 

 Canigal), — and the H. saccharata, from the Sao Lou- 

 ren90 promontory. 



In the same may we might pursue the H. erubescens, 

 and show that in the sylvan regions, and on the low 

 barren Ponta Sao Lourenjo of Madeira, on the Pico 

 de Facho of Porto Santo, on the Ilheo Chao, on 

 the Central Dezerta, and on the Bugio (where it at- 

 tains a gigantic size), it has its distract and permanent 

 phases,— the evident results of isolation, and other topo- 

 graphical influences, since the subsidence of the inter- 

 vening tracts. And in like manner, the Clausilia delto- 

 stoma is universal throughout the Madeiran Archipelago, 

 — displaying, however, in Porto Santo a fixed and 

 strongly ribbed state, peculiar to that island. Thus, if 

 the examples which we previously cited tend to establish 

 the extreme slowness of the migratory movements of 

 the terrestrial moUusca across this former continent, 

 the present ones (which refer to a few exceptional 

 species of quicker self-difiusive powers) will show, no less 

 than the insects to which I have lately called attention, 

 that where sufficient areas had been overspread (before 

 the periods of subsidence) for the creatures to have 



