[Illustration: Fig. 19.--Map of the British Islands on which the geographical distribution of _Geomalacus maculosus_ is indicated in black.]
I have had occasion to mention once before an extremely interesting genus of blind Woodlouse, viz., _Platyarthrus_. Like _Testacella_, it lives underground, and also resembles it in its general range. Its distribution is therefore of particular interest. It is difficult to conceive that _Platyarthrus_, from its peculiar mode of life could have crossed any formidable barrier, such as even a narrow straits of sea. Its occurrence in Spain and North Africa indicates, therefore, that the Straits of Gibraltar did not exist at the time when it undertook the migration southward, just as the English Channel and the Irish Sea could not have been there when it wandered to England and Ireland. The species which occurs in the south of England has a wide range in Ireland, and reaches in Scotland its most northern European limit of distribution. _Platyarthrus_ is only one of the Lusitanian genera of woodlice. In Ireland--chiefly on the west coast--we also find a brilliantly coloured Woodlouse, which is absent from Great Britain, viz. _Metoponorthus cingendus_. It reappears again on the Continent in the south of France. Its range is therefore suggestive of a Lusitanian origin; and indeed, when we examine the general distribution of the genus _Metoponorthus_, we find that out of the forty-four known species, fully one-half are confined to Western Europe and North Africa.
My friend and colleague, Mr. Carpenter, informs me that among the Irish Spiders he is acquainted with, the following are to be looked upon as Lusitanian species:--
Dysdera crocota. Oonops pulcher. Tegenaria hibernica. Theridion aulicum. Lasæola inornata. Agroeca celans. do. gracilipes. Teutana grossa. Cnephalocotes curtus. Porrhomma myops.
Of the _Coleoptera_, the genera _Trichis_, _Glycia_, and _Singilis_, all belonging to the Running Beetles (_Carabidæ_), are almost confined to the Spanish peninsula.
The beetles _Rhopalomesites Tardyi_, _Eurynebria complanata_, and _Otiorrhynchus auropunctatus_ also belong to this fauna, as also the Earthworms _Allolobophora veneta_ and _A. Georgii_, and the Millipede _Polydesmus gallicus_.