756 SIR HENRY H. HOWORTH OX THE [NoV. 9, 



its name of trunccUa. This feature is exaggerated in a vai-iety 

 found in the Uddevalla shell-beds and known as uddetalleiisis, in 

 which the shell is cut down to only half its normal size. 



This truncation was apparently not a primitive feature of the 

 species, for in young specimens it is hardly marked at all (text- 

 fig. 240), nor is it present in the striations marking the stages of 

 growth of the older shells in their earlier stages. Jensen was the 

 first to point out clearly the important fact that the contours of 

 the two species are really secondary and unimportant featui'es 

 compared with the character of their hinges, which he minutely 

 describes. This can be better seen from the figures annexed 

 (text-figs. 241-243), which he has kindly permitted me to re- 

 produce from his epoch-making paper ])ublished in 1900 in the 

 Yidensk. Meddel. naturhist. Foren. i Kjobenhavn, p. 133. 



Testing the specimens of Myas which are contained in the 

 Northern Museums by this character of the hinge, he was able to 

 show that all the Myas which from their oval outlines had been 

 treated as M. arenaria, and which had come from Iceland, Green- 

 land, Spitzbergen, Nova Scotia, the Kara Sea, and Siberia, are 

 shown by their hinges to be really M^/a truncata and not M. aren- 

 aria, and he accordingly gave them the name of M. truncata, var. 

 ovata. This diseoveiy, which has been fully accepted by Brogger 

 and other unimpeachable judges, was very important, since it 

 was on the evidence of these Arctic specimens that M. arenaria 

 had been treated as a typically Arctic shell. 



Not only so, but the alleged presence of M. arenaria in certain 

 shell-beds in Britain had in many memoirs and books devoted to 

 the cultivation of extreme glacial views been treated as a very 

 special touchstone of glacial conditions. All this will now have 

 to be revised as will the labels on many museum specimens. 

 Jensen's emphatic statement, which I will quote in his own 

 words, is conclusive: — " Resvilttatet af den forudgaaende TJnder- 

 sogelse kan i al Korthed udtrykkes saledes at Mya arenai-ia ikke 

 er nogen hojnordisk art" {op. cit. p. 149). 



What is plain, therefore, is that Mya arenaria is in no sense an 

 Arctic shell but only a boreal one, and that G. Jeffreys was quite 

 mistaken when he made it so, and when he made the further 

 inference, which has been copied into several geological works 

 and is contained in the following sentence: — " The occurrence of 

 this circumpolar shell-fish so near the tropic of Cancer probably 

 indicates the most southern limit in space of the glacial epoch " 

 (' British Conchology,' iii. pp. 65, 66). 



It is further plain that when Mya arenaria recently invaded 

 the Scandinavian waters it could not have come from the North. 

 Did it then come from the South-west, from the British seas 

 where it abounds, or from the coasts of Belgium and Northern 

 France, where it occurs as far south as Rochelle ? The fact that 

 it does not occur further south in the Bay of Biscay and on the 

 Lusitanian coast is curious. It is not less curious that its very 

 recent history on the British coasts points to its having only 



