Zoology. 



179 



Chemnitzia rufescens, Clyde 



Natica helicoides, Orkney & North Sea 



„ pusilla, North Sea 

 Velutina flexilis, „ 

 Trichotropis borealis, South of Scot- 

 land 

 Fusus berniciensis, North Sea 



Fusus Norvegicus, North Sea 

 „ Turtoni, „ 



Trophon clathratus, Irish Sea 

 „ Barvicensis, North Sea 



Mangelia Trevilliana, „ 

 „ nana, Orkney 



Philine quadrata, North Sea 



extending only to the British 



Rissoa Zetlandica 

 Skenia planorbis 

 Scalaria Trevilliana 

 Aclis nitidissima 

 Eulima bilineata 

 Natica Montagui 

 Buccinum undatum 



„ Humphrey sianum 



„ Dalei 

 Fusus Islandicus 



„ propinquus 



„ antiquus 

 Mangelia rufa 



„ turricula 



The following are northern species, 

 channel, or but little to the south of it. 

 Xylophaga dorsalis 

 My a truncata 



„ arenaria 

 Thracia villosiuscula 

 Cochlodesma praetenue 

 Tellina pygmaea 

 Cyprina Islandica 

 Astarte compressa 

 Modiola Modiolus 

 Leda caudata 

 Megathyris cistellula 

 Chiton ruber 

 Lacuna pallidula 



„ vincta 



„ crassior 



Crenella discors, I have never met with south of the British seas, and 

 suspect that when reported from the south of Europe, it has been con- 

 founded with Crenella marmorata, and Crenella costulata. Philippi's 

 description evidently applies to the former. 



It is a most remarkable fact connected with the distribution of land 

 shells, that some species are extended over very wide districts, while 

 others are restricted to an area of a few square miles, or even less. 

 Great Britain does not offer for observation a single species which is not 

 likewise an inhabitant of France or Germany, though the neighbouring 

 countries of the continent possess some which are not to be met with in 

 this kingdom ; and while thus among the hundreds of islands of Great 

 Britain not one produces a species peculiar to itself, in the groups of the 

 Canaries, Madeiras, and Azores, each island presents some species sup- 

 posed to be strictly local. 



This fact is particularly striking in the Madeiras — where Madeira pro- 

 per contains but few species ; whilethe small islandof Porto Santo supplies 

 an astonishing number, in general specifically distinct from those of Ma- 

 deira, and the rocky islets called the Desertas, with difficulty accessible 

 by man, have each some peculiar forms and in great abundance. 



These facts seem to indicate that Great Britain and Ireland, including 

 the Hebrides, Orkney, Zetland Islands, &c, have at one time formed part 

 of the European continent, but that the more distant islands which I have 

 named — raised by volcanic action from the depths of the Atlantic, have 

 been each the scene of the creation of certain species which have been 

 confined within their narrow limits by the surrounding sea. 



Opposed to this idea is the fact already alluded to, that some marine 

 littoral species, I may particularly mention Littorina striata, are common 

 to West Africa, the Canaries, Madeira, and the Azores, which (as it is 

 quite impossible for littoral phytophagous animals to have travelled along 

 the bottom of the ocean) would lead us to infer that the African continent 

 had at one time extended as far west as the last-named islands, in accord- 

 ance with an opinion very ably supported by Professor Edward Forbes, 



