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SUPPLEMENT TO THE CRAG MOLLUSCA. 



Eelaugh, and Weybourn. Middle Glacial, Billockby, Clippesby, and Hopton. Upper 

 Glacial, Dimlington, and Bridlington. 



This shell has recently been identified with N. Lyalli from the north-west coast of 

 America, and by others with N. insiynis from Japan, while there is a third shell that has 

 equal pretensions to identity, viz. N. mirabilis (Adams and Reeve) ; all these having the 

 exterior ornamentation like that upon the upper part of N. Cobboldia, but none of these 

 living shells, so far as they are known, approach in size any of the fossil specimens, 

 those of A. Lyalli that I have seen being not more than half in linear dimension ; and, 

 as pointed out by me originally (' Crag Moll.,' vol. ii, p. S3), two Cretaceous species are 

 similarly ornamented. All the specimens of these Pacific shells, which I have yet seen, 

 are destitute of that broad exterior belt extending from a fourth to a third of the shell's 

 diameter, which is free from the oblique or zig-zag markings, and forms the margin of 

 the full-grown specimens of N. Cobboldice from the Red Crag at Butley, and from the 

 successive Glacial beds of East Anglia. In order to show this belt I have had a specimen 

 figured from Belaugh. 



If the recent shells which I have seen and which are all destitute of this plain belt 

 should prove to be only young individuals, which, when full grown, would acquire it, 

 N. Cobboldics would, nevertheless, not agree with them, because the tumidity and more 

 elevated umbo of all these Pacific shells is such, that if they grew to the size necessary to 

 add on the belt they would assume a very different form than that of Cobboldia ; and 

 these recent shells are also more angular than our fossil. I, therefore, still retain this well- 

 known fossil as a distinct species. 



No trace of this shell has yet occurred in that part of the Red Crag which exists at 

 Walton Naze and which I regard as the oldest part of that formation, and as possessing 

 Mediterranean affinities, but it gets more common in the newer portions of the Red Crag 

 with northern affinities. In the Butley Red Crag Cobboldia is common, as it is also at 

 some localities of the Fluvio-marine Crag and of the Chillesford bed. In the Lower 

 Glacial, sands it is rare where these sands are Fluvio-marine, as at Belaugh, but 

 commoner where they are more marine, as at Weybourne. It is very abundant, in a 

 fragmentary condition, in the Middle Glacial sands, and it appears to be characteristically 

 the shell of the Pre-glacial and earlier Glacial periods, and to have disappeared from 

 British seas towards the later part of the Glacial period. 1 



The shell called Nucula (Acila) Lyalli, by Mr. Bell (' Ann. and Mag. Nat. Hist.,' 

 1871), was placed in my hands, and I believe it to be only the young state of Cobboldia. 



1 In the foot-note to p. 2(i of the Introduction to this Supplement, mention is made of a thin band of 

 sand intercalated in the upper glacial clay at Dimlington, which contained mollusca with valves united. 

 Since that note was published some specimens from this bed were kindly forwarded by Sir Charles Lyell, 

 and among these were several of Nucula Cobboldicc. Mr. Leonard Lyell's description, which Sir Charles 

 forwarded with the specimen, speaks of this sandhand having been literally packed with perfect specimens 

 of Nucula Cobboldicc. 



