294 Proceedings of the Royal Irish Academy. 



America, a third in Mexico, wliile still another has found means to 

 reach Madagascar. Similarly, Philoscia^ of which there are a number 

 of European species, has two in IS'orth America, ten species in South 

 America, one in Madagascar, one in Zanzibar, and another in Borneo. 



There is, however, another group of Crustacea which yields such 

 decisive indications of the former land-connection between Africa and 

 South America that scarcely anything else is needed to put that theory 

 on a firm basis. The group referred to is that of the fresh-water 

 Decapods (cf. Ortmann, A), the species on both sides of the Atlantic 

 showing a most remarkable affinity. 



There are, in the first place, to be mentioned two species of Atya, 

 A. scabra, occurring in Central America, the West Indies, and also 

 on the Cape Yerde Islands, off the west coast of Africa; while A. 

 gahonensis inhabits the Orinoco river in South America, and the 

 Gabun river in West Africa. IN'ext wo have two species of fresh- water 

 Pala3mon — viz., P. jamaicensis and P. olfersi, the former being known 

 from South and Central America, southern California, and the West 

 Indies, and also from Liberia, the Mger river, and the Congo, in West 

 Africa. Palmmon olfersi has been recorded from the West Indies, 

 from Brazil, and from the island of St. Thomas on the east coast of 

 Africa. 



Dr. Ortmann points out that the crustacean fauna of the East- 

 American literal region exhibits a very marked relationship to that of 

 western Africa — such species as Remipes cubensis, Ccdappa marmorata, 

 and Ccdlinedes diacanthus occurring on both sides of the Atlantic. 

 This fact appears to me to be suggestive of a former coast-line having 

 existed across the Atlantic, along which these shallow-water forms 

 migrated from one country to the other. Dr. Ortmann, however, 

 is of opinion (B, p. 84) that the larvae of these species had been able to 

 cross the Atlantic by means of ocean currents. 



Worms. 



There is one more group of invertebrates to be considered, 

 which is of importance in deciding questions of former geographical 

 revolutions — viz., that of the earthworms. Land-planarians, no doubt, 

 might be most useful in aiding us to unravel problems of zoogeography ; 

 but theu- distribution is as yet very little known, and their relation- 

 ship has not been sufficiently studied for this purpose. 



The ocean, according to Mr, Beddard, is an insuperable barrier to 

 most of the earthworms, and more effective than any other. The 

 latter' are therefore, as that author remarks, exceptionally qualified 



