XX INTRODUCTION. 



pilidium Ancyloide (probably arctic) ; a few species of 

 Missoa, and of so-called Shneee, Stylifer Turtoni, Syndos- 

 mya tenuis and Thracia villosiuscula, the two last pro- 

 bably much more diffused. Species well known in a few 

 localities elsewhere, but most abundant in the British seas, 

 such as Trochus Montagui and millegranus, Scalaria Tre- 

 velyana, Natica Montagui., Astarte triangularis, and Pecten 

 tigrinus and striaiits, might possibly be cited with even 

 more propriety as representatives of a British type. 



VI. A considerable number of our marine Molhisca are 

 sufficiently common on the western coasts of Britain, 

 though scarce in the Irish Sea, and for the most part 

 absent from the German Ocean, or, at least, from the 

 more central portions of our eastern coast. Most of these 

 have a considerable range to the southward of the British 

 Islands. Some of them, however, such as Trochus iinihi- 

 licatus, are confined to the oceanic coasts of Europe. The 

 whole assemblage seems to have a spread, in part indicative 

 of an ancient trend of land westwards, and in part of the 

 course and influence of the extension of the Gulf stream. 

 This assemblage may be styled the Atlantic type. Many 

 instances may be adduced, such as Akera hullata, Area 

 ietragona, Ceriihiopsis tuherculare^ Ceritldum adversum 

 and reticulatum, Circe minima, Fissurella reticulata, Iso- 

 cardia cor, Lima Mans and suhauricidata, Lucina spini- 

 fera, Mangelia attenuata, gracilis and purpurea, Modiola 

 tulipa, Natica sordida, the species of Nectra 9, Nucula 

 decussata, Nucula radiata ?, Pinna pectinata, Psammohia 

 costidata and tellinella, Rissoa lahiosa, rufilahrum and 

 vitrea, Scalaria clathratula, Solen marginatus, Solecurtus 

 candidus and coarctatus. Tapes a.urea and decussata, Tel- 

 Una halaustina and incarnata, Terebratula caput serpentis. 

 VII. The few Pelagic Mollusca that are driven towards 



