Sir H. H. Hoioorth — The Baltic — The Litorina Sea. 313 



In addition to these molluscs we may mention as also living in 

 the Eastern Baltic the Bryozoan Membranipora pilosa (var. mem- 

 branacea). Among the molluscs here enumerated two are very 

 recent additions to the Baltic fauna, and have never occurred in the 

 raised beaches or shell-beds. These are Mya arenaria (the ' clam ' 

 of the American writers) and Balanus improvisus. The Mya is now 

 quite a common shell in the Eastern Baltic, and Lyell as long ago as 

 1835 noted the remarkable fact that it was not found in any of the 

 shell-beds (Phil, Trans., 1835, pt. 1). It has no doubt migrated 

 from the Cattegat, and it is singular that it did not do so before, for, 

 as G. Jeffreys says, " it is found living in sand at low water, chiefly 

 in estuaries where there is an admixture of fresh water with the 

 sea." He says further, " its capability of living in brackish and 

 even fresh water is well known " (Brit. Oonch., iii, 65, 66). 

 The second recent addition is the acorn-shell, known as Balanus 

 improvisus, which has occurred at Reval, Helsingfors, and in the 

 Gulf of Finland, and may have been introduced on ships. 



Putting aside these two recently imported forms, which are 

 irrelevant to the purpose of this paper, the other species above 

 mentioned are all found in the shell-beds and recent deposits of the 

 coasts of the Eastern Baltic. 



In regard to the effect which the slight salinity of the waters of 

 that sea has had on the size of the shells, Munthe has shown how its 

 mussel (which is true also of its cockle) diminishes rapidly as the 

 water becomes fresher. Thus he gives the normal length of the 

 mussel in different parts of the Baltic in the following table : — 



mm. 



Kiel 110-0 



Bay of Travemunde ... ... ... ... ... SI'O 



Between !^oeii and Falsterbo ... ... ... ... 64*0 



Kiigen 52'4 



Gotland (probably too little) c. 40*0 



Stockholm Archipelago 43'0 



Aland c. 35-0 



Ulfon, the innermost known Swedish locality ... ... 22"5 



Stanggrund, the innermost known Finnish locality ... 21*0 



It seems clear that the addition of fresh water to salt in more than 

 certain proportions inevitably dwarfs or distorts the shells, and the 

 deformity is to be found wherever marine molluscs are living under 

 brackish conditions. It is due probably partially to the want of 

 adequate food for the mollusc itself, and partially to the absence in 

 the brackish water of sufficient adequate materials to build up the 

 shell. 



Let us now turn from the recent shells in the Eastern Baltic 

 to those found in the later shell-beds on its shores. 



The first person to recognize these shell -beds was Magnus 

 Bromell, who speaks of an accumulation of Mytiliis and Cardium 

 shells near Stockholm (Lith. Suec, specimen secundum, Upsala, 

 1727, p. 58 et seq.). 



Linnaeus, in the first edition of his " Systema Naturje," 1735, 

 refers to the Mytilus accumulations in Helsingland as " humus 



