O CATALOGUE OF THE MOLLUSCA 



But if our Catalogue exclude some of the objects usually asso- 

 ciated with the Mollusca, it will, on the other hand, be found 

 more than usually extensive in two classes, which have seldom 

 obtained a place in our local lists, namely, the Nudibranchiata 

 and the Tmiicata. In the latter so little has hitherto been done 

 that, even with the kind assistance of of our friend Professor Ed- 

 ward Forbes, we have found the greatest difficulty in referring 

 our species to any of those already described, and it has conse- 

 quently become necessary to publish descriptions of many of them. 



The coast over which our observations extend, is in some re- 

 spects not particularly favourable for the production of molluscous 

 animals : possessing none of those sinuosities and sheltered bays 

 within which these animals generally find the conditions most 

 favourable to their development. The coast line of Northum- 

 berland and Durham is, on the contrary, bare and exposed, and 

 the sea beats upon its shores much more roughly than in most of 

 the localities celebrated for their marine productions. It is 

 nevertheless not without some advantages. Its extensive sands 

 are unmixed with shingle, and consequently afford a suitable 

 habitation for many bivalves ; which, however, are nearly ex- 

 tirpated on some of the most frequented parts of the coast 

 From the nature of the strata, too, the rocks do not dip very ab- 

 ruptly into the sea, but run out at a very slight inclination 

 leaving a great extent of surface at low tides, accessible to the 

 naturalist, and not unproductive of marine animals. In the 

 inland portion of the country the surface is a good deal diver- 

 sified, but upon the whole it is rather too hilly, particularly in 

 the western parts, to be very productive of the land and fresh- 

 water Mollusca, which are generally thinly scattered in upland and 

 exposed situations. The vallies, however, produce not an incon- 

 siderable number of the land species : these abound most on 

 limestone, and a few species are nearly confined to that descrip- 

 tion of rock. Owing to the small extent of marshes and ditches, 

 in addition to the lower temperature, the fresh-water species are 

 much less abundant here than in the south of England. 



The generalizations of Professor E. Forbes, in his able Paper on 

 the Northern Drift, published in the " Memoirs of the Geological 



