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IRotea on (Br^ba^a incurva. 



By John French. 



T'HESE are the commonest fossils of the Essex Boulder Clay. 

 They are vulgarly known under the name of " DeviFs Toe- 

 Nails," and are familiar to almost every ploughboy. Like 

 very many others of their colleagues, they have been subjected to 

 extensive abrasion and do not often occur as perfect fossils. The 

 geological range of these fossils is from the Lias to the Chalk 

 (both inclusive), but so far as I know Liassic examples do not 

 occur in Essex. Our derivatives, I believe, are principally from 

 the Oolitic series (Oxford clay, etc.). The abrasion, for the most 

 part, must have long anteceded the advent of the Boulder Clay, 

 for most of the examples, broken from their matrix of hard rock, 

 show considerable wear. It seems to be the fate of this shell 

 generally to appear under this aspect, even in Liassic specimens. 

 The amount of wear and tear and tossing about can also be 

 inferred by another standard, and that is the general absence of 

 the right or upper valve. Perhaps I should be within the truth if 

 I said that not one specimen in a hundred bears this appendage. 

 I could also safely say that not one specimen in a thousand (from 

 the Essex Drift, at least) is a perfect fossil. It has been my good 

 fortune to meet with a few such examples, one of which was 

 obtained from the Boulder Clay at Felstead, and is the one on 

 which I shall venture some remarks. 



In a much-hardened form, every minute ridge and marking of 

 the original shell is preserved in this specimen. Moreover, the 

 original had the advantage of belonging to a healthy animal, 

 which reached maturity without misadventure and without the 

 disadvantages attending overcrowding, and so was enabled to 

 produce a shell as regular as the species would allow of, and as 

 perfect as any healthy mollusk ever did. 



To say that the animal " kept the even tenour of his way " 

 would be stating a case to which probably no mollusk responds. 

 There is sufficient evidence here of quite another state. There 

 are main ridges in the shell corresponding to seasonal growths, and 

 secondary ridges corresponding to minor growths. Like the 



