762 SIR HENRY H. HOWORTH ON THE [I^ToA". 9, 



coast of America, wliei-e it occurs as far south as Carolina and north- 

 wards to Lahiador. Brogger classes it in fact with those other 

 shells in the Christianiafiord which are also recent immigrants and 

 have been very pi-obably deiived from America, e. g. Acvicea testudi- 

 nalis, Loj)hyri(,s alhus, Seal-aria groenlandica, Cerithiopsis costulata, 

 and Nucula delphhiodonta (l^rogger, op. cit. p. 595 &c. and p. 712). 



The history of Mya arenaria in America is a curious one. Its 

 old home there is on the Atlantic coast of Canada and the United 

 States, where it is a favourite edible mollusc, known as the 

 Clam, but it has quite recently (about 1874) been transported to 

 California to be fattened and has since spread rapidly in San 

 Fi'ancisco Bay (see Stearns, Mya arenaria in San Francisco Bay, 

 American Naturalist, xv. p. 362). 



Let us, however, return to Europe, for we have not yet exhausted 

 the interest of this shell. 



While it seems plain that 3Iya arenaria is not found in any of the 

 deposits in Europe later than the drift except in the curiently 

 deposited alluvium, there can be no doubt that it occurs, and occurs 

 abundantly, in the Crag beds both in the Red or Suffolk Crag and in 

 the Upper or Norwich Crag and its several divisions. It is quoted 

 by Whitaker from the Red Crag at Beaumont (Mem. on Sheets 

 48 S.E. and 48 JSF.E. p. 30). It is also named from the Crag at 

 Bulchamp Pit, Dunwich Cliff, and Southwold (Geology of South- 

 wold, p. 83), from the so-called Chillesden Beds at East Barent 

 by Prestwich (Q. J. Geol. Soc. xxvii. p. 345), and from the so-called 

 Weybourne Crag at Trimingham, Sidestrand, Overstrand, Runton, 

 Sheringham, and Weybourne, and generally as very common by 

 Mr. Clement Reid ; and locally from the ISTorwich Crag at Burgh 

 near Aylsham by Mr. Harmer, who in another place reports the 

 shell as common in the Norwich Crag (Proc. Geol. Assoc, Later 

 Tert. hist, of East Anglia, p. 466). 



It is also reported from the shell-bed on the shore at Selsea 

 (sen R. Bell, Yorks. Phil. Soc. 1892). This bed is clearly older 

 than the drift, that is to say, is what is generally called pre-glacial, 

 since it underlies the famous gravel-bed which contains very la.rge 

 boulders. G. Jeffreys also quotes it from the Crag of Belgium. 



In regard to the Chillesford beds there are some fine, perfect 

 and very typical examples of the true Mya arenaria in the British 

 Museum. 



It is perfectly plain, therefore, that Mya arenaria was living 

 in the Biitish Seas in the period of the Red and Norwich -Crag, 

 as it is plain that it is an abundant shell on our coasts now, but 

 that between these two periods it became extinct here and 

 was reimported. This involves some interesting issues. The 

 only marine beds lying between the Norwich Crag and the raised 

 beaches are the drift or so-called glacial beds. What is the 

 testimony of these beds on the question, and what is the exact 

 meaning of that testimony ? A very great change has come over 

 geological opinion in regard to the fossil contents of the drift 

 beds during the last thirtj' years. It was Searles Wood who first 

 separated the so-called Middle Sands of Norfolk fiom the Ciag. 



