214 Alfred Bell — The Succession of the Crags. 



The difference in age will aceonnt for the absence of the typical 

 forms {i.e. Southern and Coralline) of the older areas from this. 

 The abundance of Littorina and Turritella alone marking the Trans- 

 Iken area, and that only in certain places. 



In as few words as possible, I have endeavoured to show that a 

 great difference does obtain in the Eed Crag marine fauna, marking 

 two distinct geological horizons. I have confined myself to the 

 Mollusca, although in every class of animal life the same differences 

 prevail more or less. It is further in proof of my statement, that 

 the only terrestrial mollusca of the lowest Eed Crag are of an extinct 

 species, those of the newer division living in our own land at the 

 present day. 



The CMllesford Sands and Clays. — If the " unproductive sands " 

 which cover the whole of the Coralline Eed and Norwich Crags 

 are of Chillesford age, I am in error in considering the totally un- 

 fossiliferous sands capping the Shelly Crag at Butley (see Geol. 

 Mag., 1871, Vol. VIII., p. 450) as of different age; but as 

 the whole of these sands, whether in Suffolk or Norfolk, are un- 

 fossiliferous, I must still be content to retain them as older, or 

 rather that when the denudation of the Crag area commenced (so 

 well seen at Butley), the " unproductive sands " were thrown down, 

 as we always find that below the Chillesford shell-bearing sands, 

 whether at Chillesford, Sudbourn Church Walk, Aldeby, Norwich, 

 or elsewhere, these sands invariably underlie them, sharply making 

 a distinct horizon and a different condition of things. 



I cannot assign the Bawdsey Cliff fossils listed by Mr. Prestwich 

 to this stage. In mode of deposition, in, character, and in species, 

 the shells are the same as in those of the Upper Crag area (of A. 

 and E. Bell). 



The uncertainty as to the value of the " argillaceous " zone, and 

 the disagreement as to its range among geologists, renders the age of 

 the Forest bed a still open question ; but I am happy to be able to 

 supplement Mr. Prestwich's list of shells from Eunton by the follow- 

 ing species, either obtained by Mr. Eeeve or myself : SpTKerium 

 rivicola (Kessingland), Pisidium amnicnm var. sulcatum, P. fontinale, 

 P. ITenslotviana, TJnio liUoralis, Valvata antiqua, Planorbis corneus, 

 P. nautileus, P. nitidus, Limnea pereger, Ancylus fluviatilis, Physa 

 fontinalis, Succinea putris, Helix Jiispida, H. arbustorum, Carychium 

 mini^niim, Zua luhrica, Limax agrestis. 



The name Unio margaritifera in Mr. Prestwich's list is added 

 through an error. The specimen referred to, which is in the Norwich 

 Museum, is an Anodon. 



There are two exceptions I must take to the lists of Mollusca, 

 which bear upon the per-centage question. I notice that many 

 extinct forms are referred to existing species, or else they are 

 termed varieties. How this can be, I am at a loss to understand, 

 when the so-called variety lived, as far as we know, in advance of 

 the typical form. Taking an extreme case, Tellina Benedenii, which 

 first appears in the Black Belgian Sands, is termed a variety of 

 Tellina lata, which first appears in the " Sables gris," a deposit 

 ;posterior to the Coralline Crag. 



