BRITISH NUDIBRANCHIATE MOLLUSCA. 29 



similar to what are found on the opposite coast of England. Concerning the shores of 

 Holland and western Germany, our information is very scanty, but as they are generally low 

 and sandy, the Nudibranchs found on them are probably few. Those of Bomme's species 

 that have been recognised are identical with our own. As we approach the north of Europe 

 our information concerning these animals becomes more extensive. At least two thirds of the 

 species in the Scandinavian fauna agree specifically with those of Britain, and, when both 

 countries are more completely searched, it is not unlikely that this agreement may be found 

 yet more intimate. 



The distribution of the British species will be best understood by an examination of the 

 following tables, in which we have endeavoured to condense all the information on the subject 

 we at present possess. 



It has been considered sufficient, in the table of Geographical Distribution, to adopt the 

 natural and obvious division of the shores of the British Islands into three regions — southern, 

 eastern, and western — forming the three sides of a triangle whose apex is the north. The 

 northern forms may by this means be somewhat divided; but this division is naturally 

 suggested by the form of the coast line, and we the more willingly adopt it, as so little is 

 known of the littoral Nudibranchs inhabiting the extreme north-western shores. The true 

 northern marine fauna of our Islands will be found on the north-eastern coast, extending from 

 Shetland to the Humber ; a circumstance arising from the set of the tide from the north in 

 that direction. Scarcely anything is known of our Mollusca from the Humber to the Straits 

 of Dover. Properly speaking, therefore, the three known divisions of our shores resolve 

 themselves into the southern, the western, and the north-eastern. The southern coast is the 

 richest in specific and generic forms, more especially the latter. Seven genera inhabiting the 

 southern and western coasts are not found on the eastern : these are Thecacera, Scyllcea, 

 Lomanotus, Fiona, Jlderia, Jntiopa, and Proctonotus. Not a single genus of the latter division, 

 however, is absent in the other two, though Embletonia ought perhaps to be considered a 

 northern form. As has been observed in other departments of the Mollusca, the southern 

 species extend much further northward on the western shores of Great Britain than on the 

 eastern. This arises from the set of the currents on that side of the island in a northerly 

 direction ; and the influence of the Gulph Stream on the western shores of Ireland and 

 Scotland seems to have a similar effect. On this account probably it is that two or three 

 species inhabiting our southern and western shores, though not found on the east coast, 

 make their appearance again on the western shores of Norway. 



In the table of distribution according to zones of depth, we have adopted the three great 

 divisions of littoral, coralline, and deep-water. The laminarian zone, extending from 

 low-water mark to a few fathoms in depth, we consider to be a sub-region belonging to the 

 littoral, and characterised mainly by the same species, nearly all the inhabitants of the 

 Laminarian zone being occasionally found within tide-marks. The two together constitute 

 the region of sea-weeds or Alga. It would be easy to subdivide this region into smaller 

 zones, each characterised by one or two particular species, but we have thought it unnecessary 

 to do so in the present instance. The limits of each species, as far as they have been 

 ascertained, are mentioned in the specific descriptions. A glance at the table will show that 

 the greatest number of species is to be found in the littoral zone; though the number 



