4 WATSON : RELATION OF MADEIRAN MOLLUSCA TO OTHERS. 



immigrants. Of these there are four : Belix {Hyalinid) crystal- 

 Una Miill., ff. {Pyramidiild) pygmcea Drap., ff. {Euparypha) 

 pisana Miill, H. {Caracollina) lenticula Fer. 



6. Indigenous Atlantic species which are neither European 

 nor Mediterranean. Of these therearesix: HeIix{Patula)placida 

 Shuttl., H. (P.) pusilla Lowe, H. {Leptaxis) eruhescens Lowe, 

 H. {Spirorbuld) paupercula Lowe, Pupa ( Gastrodon) fatiaknsis 

 Lowe, P. {Trimcatellina) viicrospora Lowe. A group this of 

 perhaps the greatest interest and the most instructive of all, 

 but which at present may simply stand as they are. 



One hundred and sixty-one species then are the true Terrestrial 

 native shells of the Madeiran Islands, and of these twenty-eight 

 species or a Utile less than one-sixth are found elsewhere. 

 Twenty-eight species out of one hundred and sixty-one ! That 

 is the whole extent to which we can say that the Terrestrial 

 moUusca of Madeira and elsewhere overlap : and one hundred 

 and thirty-three species have not only had their origin in, that is, 

 are autochthonous for this little group of islands not four 

 hundred square miles in extent, but have never spread further. 

 That of itself is a very remarkable fact deserving of careful 

 notice, but there are circumstances connected with the history of 

 the island which greatly enlarge our intelligence of this fact. 



It was in a.d. 1418 that these islands were discovered, 

 and since that period no change of any importance in the con- 

 dition of the islands has occurred. There has been neither 

 elevation nor depression nor volcanic outbreak of any kind during 

 these five hundred years. Anterior to that period, however, 

 probably long anterior, there must have occurred changes great 

 at least, if not violent — changes of level — variations on the coast 

 line — exhalations of mephitic vapour — outbursts of heated 

 water, which, pouring down the wooded slopes, both killed and 

 swept away in myriads the mollusca whose shells, now semi- 

 fossil, often crushed to atoms, often perfect, mixed with sand, 

 and piled in places many feet deep, cover square miles of 

 Madeira and of Porto Santo. Other beds too, more clayey in 



J.C., vii., Jan., 1892 



