INTRODUCTION. XVll 



found both to the south and to the north of it. But such 

 types, mostly of southern origin, can be traced in the 

 course of their migration along the Atlantic coasts of Ire- 

 land, where their progress northwards has been favoured 

 by the genial influence of warm currents. The most un- 

 productive district is the southern half of the eastern coast, 

 Our marine Molluscan fauna, when considered with 

 respect to its home arrangements, may be said to be com- 

 posed of examples of no fewer than nine types ; — 



I. The LusiTANiAN type, exemplified in species which 

 are to be met with only in the extreme south, members of 

 the Lusitanian and Mediterranean faunas, whose northern 

 limits just impinge upon our area. Haliotis tuherculata., 

 Murex corallinus, Lachesis minima, Rissoa lactea and 

 striatula, Pandora rostrata, Donax politus, Ervilia cas- 

 tanea, Lucina divaricata, Mactra h^lvacea, Avicula Taren- 

 tina, Galeomma Turtoni, Trochus striattis, Trimcatella Mon- 

 tagui, and Onchidium Celticum, may be cited as the prin- 

 cipal representatives of this assemblage. 



II. The South-Bkitish type ; to this belong species of 

 southern origin with a somewhat wider diffusion than the 

 last, occupying a limited but well-marked range along the 

 southern and south-western coasts of England, and southern 

 and western shores of Ireland, many of them entering St. 

 George's Channel, but few passing the confines of South 

 Wales. Such are Adeorbis subcarinata. Bulla hydatis, 

 Cardium aculeatum and rusiicum, Chiton discrejyans, Cy- 

 therea Chione, Dentalium Tarentinum, Diodonta /ragilis, 

 Diplodonta rotundata, Emarginula rosea, Gastrocheena Mo- 

 diolina, Lepton squamosum, Modiola barbata, Nassa pyg- 

 mwa, Ovula patida, Petricola lithophaga, Pholadidea papy- 

 racea^ Rissoa costulata, Trochus exiguus and lineatus, 



Venerupis Irus, and Venus verrucosa. 



c 



