INTRODUCTION. XXI 



or habitually frequent our seas are also to be found mainly 

 upon oiir western boundaries. They constitute an Oceanic 

 type. Our native and occasional Pteropods, as Hyalcea 

 trisjpinosa, and the three species of Spirialis ; our few 

 pelagic Gasteropods, as lanihina communis, exigua and 

 pallida, and, just possibly, Scissurella crispata ; the Nu- 

 dibranchous Mollusk, Scyllwa pelagica ; the tunicated 

 swimmer, Salpa runcinaia ; borers in floating wood, such 

 as the various species of Teredo and the Xylopliaga dor- 

 salis ; and, lastly, our twelve species of Cephalopods may 

 be ranked in this interesting group. 



VIII. We have before remarked that in the northern 

 division of the British Seas there are many species either 

 not found more to the south, or else becoming rarer as we 

 proceed southwards. Some of them, too, are only to be 

 met with in peculiar and limited localities, grouped toge- 

 ther like isolated colonists. The majority of these north- 

 ern forms belong to an assemblage that constitutes the 

 Boreal type of our Molluscan fauna. They are all species 

 that thrive best in seas to the north of Britain, and many 

 of them range across the Boreal Atlantic, or, at least, are 

 found on both sides, but only within cold waters. They 

 are not, however, to be considered as strictly Arctic. To 

 this group we may assign such examples as Acmaa tesiudi- 

 nalis, Astarte coinpressa and elli2)tica, Cardium Suecicum, 

 Cerithium metula, Chemnitzia fulvoclncta ? Chiton Han- 

 leyi and marmoreus. Crania anomala, Crenella nigra, 

 Cyprina Islandica, Emarginula crassa, Fusus Norvegicus 

 and propinquus, Hypothyris psittacea, Leda caudata and 

 pygmcea, Mangelia nana, Natica Helicoides, Nucula tenuis, 

 Panopa.a Normgica, Pecten Danicus? Philine quadrata 

 and scabra, Pilidium fulvum, Puncturella Noachina, Syn- 

 dosmya intermedia, Thracia convexa^ Tricliotropis borealis, 



