THE EUROPEAN SEAS. 



213 



certain characters. They are generally ribbed, the 

 edges of the valves are crenulated and interlock ; in 

 the great majority of the species the valves shut 

 close. The hinge consists of two central teeth in 

 each valve, and two lateral, somewhat removed ; in 

 all, four teeth in each valve. The common Cockle 

 (Cardium edule) is a good type of the genus. 



The Cockles are mostly marine, but our common 

 edible species is found in harbours and high up 

 tidal rivers, where the water becomes brackish ; in 

 these cases the shells present several modifications : 

 they are invariably reduced in size, are thin, and have 

 their external characters less strongly marked. The 

 Baltic Cockle (C. Balticum) presents such changes, 

 as do also the Black Sea and the Caspian form of 

 this species. 



The shell known as C. rusticum (Chem.) is recog- 

 nised by Philippi and Middendorf as a variety of 

 the common Cockle — an aberrant variety, says Ed. 

 Forbes, produced by the admixture of fresh water 

 with the saline element. This variety is found in 

 all European seas. In the Caspian the differences 

 betwixt the C. edule and the G. rusticum are clearly 

 marked only in the young shells ; when older, they 

 become so alike as to be scarcely distinguishable. 



There is another aberrant form of Cardium, 

 known as the Greenland Cockle, which lives in 

 estuaries there, and although it no linger belongs 

 to our European area, it is met with in abundance 

 as a fossil shell in the crag deposit of Suffolk and 



