The close resemblance between the Molluscan fauna of the Coralline Crag and that of the Mediterranean at the present day has long been known and is universally recognized. It has been customary, however, to take the whole list of shells from this deposit, for the purpose of comparison, without reference to the greater or less abundance of the different species; but in discussing the affinities of the fauna from this, or indeed from any other horizon of the East Anglian Crags, it may be misleading to attach as much importance to the presence of a shell, of which only one or at the most a very few specimens have been discovered during so many years, as to the occurrence of forms which are found in such countless profusion as sometimes to compose a large proportion of the whole number of the shells present. Out of about 440 species of Mollusca from the Coralline Crag given by Mr. Wood in his well-known Monograph (excluding varieties), nearly 90 are said to be represented by unique specimens only, while more than 100 others have very rarely been met with. It is true that some of these rarer forms may be only locally rare, although it is worthy of notice that, with few exceptions, the species which are common in the Diestien beds of Belgium, believed to be approximately contemporaneous with the Coralline Crag, are also common in that deposit. On the whole, without ignoring altogether the existence of the rarer forms, it seems that a consideration of the general facies of the fauna of each of the different horizons of the Crag, and of its characteristic fossils, is more important and is likely to give more reliable results than an analysis of all the species of each bed, irrespective of the greater or less frequency with which they have been found.