258 A. ^ R. Bell— On the English Crags. 



is of as much value, both geologically and palgeontologically, as is the 

 specific type. It so frequently happens that the variety precedes, 

 co-exists with, or succeeds the received specific type, that an elimi- 

 nation of the variety from the list of a fauna is likely to lead to 

 serious complications and even errors ; e.g., to take the case of 

 Tellina obliqua, T. prcBtenuis, and T. calcarea {or proximo). The first 

 ranges upwards, from the Coralline Crag to the Upper Grlacial, or 

 Bridlington, beds, where it dies out. The second variety first 

 appears in the lower part of the Eed Crag rather abundantly, even 

 at first ; and dies out in the Chillesford series, and T. calcarea, the 

 specific type of the whole, as so suggested by Prof. Forbes, occurs 

 only sparely at first in the Eed Crag, and attains its maximum 

 development in the Post-glacial beds of the north. The type form 

 itself does not occur in the Coralline Crag. 



The points we propose discussing are — 



1st. The derivation of species from the destruction or abrasion of 

 the older deposits. 



2nd. The position of the Eed Crag with respect to the Coralline, 

 and fluvio-marine formations. 



3rd. The evidence in favour of a redistribution of the Crag areas. 



First. The derivation of species, etc. No two authors are agreed 

 either as to the number of species, or even to the species themselves, 

 which are to be regarded as extraneous. In the abstracts of some 

 papers " Upon the Structure of the Crags, etc." (Quart. Journ. Geol. 

 Soc. 1868-70), Mr. Prestwich states that all three crags contain 

 derivate forms, the Eed Crag list especially being reduced by means 

 of derivatives and varieties from 245 to 146 species. Mr. S. Wood 

 (Lyell, Students' Elements) admits 25 species as extraneous, against 

 60 in 1858. Till some harmonization of this complication takes 

 place, all endeavours to fully comprehend the fauna are useless. 



From our observations we have come to the conclusion that there 

 is no evidence whatever, either external or internal, to justify us in 

 separating from the fauna of the Eed Crag any species except where 

 such forms are of Eocene or of older date (the " box stone" fauna 

 being out of court at present). Individual specimens undoubtedly 

 do occur that are the relics of preceding stages, but we cannot find 

 any grounds for believing that all the members of any species are 

 extraneous to the fauna or deposit in which they occur. 



Again, the reasons assigned for considering any of the forms of 

 the Eed Crag derivative are by no means satisfactory, even when 

 any reasons are given, which is not always the case. In his paper 

 upon the " Extraneous Fossils of the Eed Crag," Mr. S. Wood con- 

 siders 54 species are probably derivative, and six others positively so, 

 and he remarks that " the shells may have suffered little or no 

 abrasion during the removal." No reason for this opinion is given. 



Mr. Wood, junior (On the Eed Crag, etc., of the Eastern Counties, 

 Ann. and Mag. Nat. Hist. 1864), says of the Walton deposit that it 

 is " destitute of those derivative shells, etc.," and "bivalves are fre- 

 quently found in pairs, and univalves uninjured in the pullus or 

 apex." These evidences of non-derivation are by no means peculiar 



