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SUPPLEMENT TO THE CRAG MOLLUSCA. 



Medial Tertiary shells, depends upon the correctness of the identifications themselves, and 

 this I liave not had the means of examining. 



A list of extraneous fossils was given by myself in a paper published in the 1 Quart. 

 Journ. Geol. Soc.,' 1859, but this would now require to be much added to. These are, 

 however, undoubted derivatives, but I have been greatly embarrassed with specimens 

 that do not belong, so far as I know, to any described species, and to which a strong 

 suspicion attaches that they come from some older Tertiary formation. In such cases, as 

 well as in those of known older Tertiary species, where the mineral condition of the 

 specimen does not of itself indicate that it is derivative, I have figured the specimens, but 

 in the synoptical list which follows I have distinguished all such as clearly derivative. 

 There are besides these many shells in the Red Crag which I am disposed to regard as 

 only present in that formation as derivatives. 1 The number of these is greater than was 

 the case when the ' Crag Mollusca" left my hands in 1856, and I have distinguished them 

 in the list as possibly or probably derivative. In one instance I have been compelled to 

 regard as a genuine fossil of the period what I had before treated as derivative. This is 

 Voluta Lamherti. In other instances, such as Cassidaria bicatenata and Trophon elegans, 

 which were then only known from the Red Crag, I have satisfied myself not only that 

 they are Cor. Crag species, but that their presence in the Red is due only to derivation. 

 The only part of the Red Crag which is genuine and free from derivatives is that of 

 Walton Naze (where Volida Lamherti that I had wrongly regarded as derivative does 

 occur), all the rest of the formation being more or less leavened with derivatives from the 

 Coralline Crag, and from older formations, as well as with shells from older beds of Red 

 Crag age, such as Walton, as explained at pagevii of the Introduction to this Supplement. 

 It is unfortunate that we get only this one deposit of Walton with a genuine fauna, since 

 from the change in climate and consequent introduction of many northern forms into the 

 newer parts of the Red Crag (as explained in the Introduction) the Walton fauna, though 

 genuine, does not show what was the true fauna during the later stages of the Red Crag 

 formation. 



The elimination from the Red Crag fauna of the additional derivatives from the 

 Coralline tends to increase the Palaeontological distinction between those deposits, while 

 on the other hand the discovery, since the ' Crag Mollusca ' left my hands, of some few 

 Coralline Crag species among the genuine Red Crag fauna pretty well balances this 

 increment. I, however, see no reason to modify the opinion I have always entertained as 

 to the complete separation of the Coralline and Red Crags, although the fauna of the 

 oldest part of the Red Crag has a greater Mediterranean affinity than that of the newer, as 

 pointed out by me in the 22nd volume of the ' Quarterly Journal of the Geological Society,' 

 p. 542. With respect, however, to the Red and Eluvio-marine Crags, and their over- 

 lying Chillcsford clay and sand, they can, in my opinion, be regarded as only one deposit, 

 constituting in England the upper Crag, as the Coralline does the lower j and the triple 



1 The phosphatic nodule excavation in the Red Crag at Waldrhigfield is quite a museum of derivatives. 



