100 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 
rnore than usually extensive in two classes, which have seldom 
obtained a place in our local lists, namely, the MVudibranchiata 
and the Z’wnicata. In the latter so little has hitherto been done 
that, even with the kind assistance of of our friend Professor Hd- 
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 
