1909.] RECENT BIOLOGY OF SOME LIVING SHELLS. 747 



the other species of the genus are found only where the water is 

 quite salt (Phil. Trans. 1885, p. 309). 



Although it can live where the salinity of the water is com- 

 paratively slight, and is found in brackish water in the inlets on 

 our own coast where the marine molluscan fauna is poor, it shares 

 the feature of all marine shells transported to less salty water in 

 being dwarfed and having a thinner shell. Thus in proceeding 

 eastwards in the Baltic, where the water gradually grows fresher, 

 we find its normal length in the Harbour of Kiel to be 100 mm., 

 in the Gulf of Finland 55-70, and in the Bothnian Gulf 36"5. 



The fact that so far as is known it does not occur anywhere in 

 the Baltic in a raised beach however slightly elevated, or in. a 

 subfossil condition, makes it plain that since its introduction, there 

 has been no appi-eciable elevation of the coasts of the lands 

 bordering that sea. This may be said with some confidence of 

 the period since Lyell wrote his Bakerian lecture in 1835, i. e., 

 about seventy-five years ago when we know it was living in the 

 Baltic. 



"We will now turn to the Danish waters. There the evidence 

 is equally plain that the shell we are discussing has only arrived 

 recently. In his memoir entitled " Kartbladet Skamlingsbanke," 

 describing the district on each side of the Little Belt, published 

 by the Geological Survey of Denmark in 1907, A. Jessen has a good 

 deal to say about the shells found in the north-western part of the 

 island of Funen at the entrance of the Belt. Among the shells 

 there found he mentions especially Mi/a arenai^ia as occurring in 

 two places in what the Scandinavians call Cardluin deposit, or 

 what we should call estuarine mud. These places are both situated 

 in what was lately the upper part of the Gamborg Fiord, but which 

 has been recently embanked and laid dry. The extreme recency 

 of this deposit is shown by an excavation Jessen made in the 

 soil and by the table he gives of the depths at which the various 

 shells in it occurred. The Mya only occurs in the surface layer 

 at from -40 to "75 of a metre in depth, and is not found at any 

 lower horizon in this estuarine deposit. It is plain, therefore, 

 that it has only arrived in the Little Belt quite lately. 



The only other part of Denmark in which the shell has been 

 found on dry land is in the extreme north of Jutland, on the 

 shores of the Limfiord and in " Wendsyssel," north of that inlet. 



In another Danish Survey memoir also written by Jessen. and 

 dated 1905, describing the eastern part of the Limfiord and some 

 of its islands, he publishes some interesting tables of distribvition 

 of the shells found iii the most recent beds, wdiich he classes in 

 five series. In two of them, namely the raised oyster-beds, of 

 which he describes 22, and in the lagune deposits still in progress 

 (Lagunedannelser), of which he describes 8, the Mya does not 

 occur at all. Among a series of 20 beds which are found 

 bordering fiords and sounds with a stagnant and sluggish water, 

 one only, situated S.E. of Broust, contained Mya arenaria. This 

 deposit ivas at the sea-level. At Vejlen, north of the island of 



