312 Sir H. H. Howorth—The Baltic— The Litorina Sea. 



dependent on the distribution of the mollusca, has calculated the 

 position of the isohalinic line, or line of equal saltness, for the 

 percentage of -07, -06, -05, -04:, and -03 respectively, at various 

 points of the Eastern Baltic from the island of Gotland to the 

 upper reaches of the Bothnian Gulf. 



These figures show that between the Cattegat, where the ocean 

 is close by, and the upper part of the Bothnian Gulf the degree of 

 salinity in the Baltic decreases gradually from 296 per cent, to 

 •03 per cent. This distribution of salt in its waters has naturally 

 had a great effect upon the marine moUuscan fauna of the Baltic, 

 and also has affected their size and caused them to become distorted. 

 To this interesting issue we will now turn. For the purpose of 

 investigation the Baltic may be divided into two sections by a line 

 drawn a little to the east of the island of Bornholm, and we will at 

 present limit ourselves to that section of the sea (much its larger 

 portion) lying to the east of such a line. 



The poverty of the living molluscan fauna of this part of the 

 Baltic is phenomenah That fauna is, in fact, limited to the following 

 shells : — 



(1) Tellina balthica, var. a and b. The form found in the Baltic is 



the original type. It also occurs in the Southampton Water 

 and in the Orwell estuary. Loven deemed it a degenerate form 

 of T. solidula, due to the brackish conditions in which it lives. 

 Jeffreys says it is a hardy mollusc, accommodating itself to all 

 kinds of temperature and all kinds of water, from nearly fresh 

 to the saltest. 



(2) Mytilus edulis (a dwarf form). 



(3) Carditim edtde (also a dwarf form). 



(4) Mya arenaria. 



(5) Balunus imyrovisns. 



(6) Falndinella balthica (a distorted form; it was formerly called 



Mydrobia ulva). Under the latter name Jeffreys calls attention 

 to the distinction of this species from the rest of the Jlydrobice 

 in that it loves brackish water, in which it in fact lives in the 

 Thames. Elsewhere he says that it is never found out of the 

 reach of the tide.' 



(7) Neritina fliiviatilis, of which Munthe says that it is found at 

 Landskrona in water with 1*2 per cent, of salt, that it lives in 

 slow rivers, streams, and lakes, but that a dwarf variety has 

 been described by Nilsson inhabiting the shores of the Baltic and 

 adhering to seaweeds and stones, sometimes at a distance from 

 the mouth of any river, and living in company with the common 

 mussel and a few other decidedly marine shells (id., p. 54). In 

 this case the dwarfing has doubtless been caused by the water 

 being too salt instead of too fresh for the mollusc to adequately 

 thrive. 



1 In regard to this mollusc a note of Jeffreys is curious. It seems it has been 

 found in salt springs and marshes in the Ain and Jura, and he thinks this may 

 indicate there an ancient estuary ot the Ehone (Brit. Con., v, pp. 208-9). 



