THE EUROPEAN SEAS. 



203 



dividuals as compared with their representatives in 

 the Mediterranean or Atlantic. 



To these forms must be added Dreissena poly- 

 morpha, which has now established itself in most of 

 the rivers of Western Europe, but of which the na- 

 tive home is in this region, and the dreaded Teredo 

 navalis. The abundance of this Mollusk in the 

 harbour of Sebastopol is so great, and the destruc- 

 tion of the vessels which it attacks so rapid (eight 

 years being the average duration of the under 

 timbers of any ship), that it is possible that a 

 great service was rendered to the naval power of 

 Eussia when it was compelled to withdraw its fleet 

 from the Black Sea waters. 



Some peculiar forms of Cardium occur in parts 

 of the Black Sea, and which are common to the 

 Caspian. Cardium plicatum is found at the mouth 

 of the Dnjestr, and C. color atum at that of the 

 Dnjepr and the Don. 



The deficiencies in the Black Sea fauna are re- 

 markable. All those classes of Mollusca which, as we 

 have seen, are but poorly represented in the East- 

 ern Mediterranean as compared with the Western, 

 are here either altogether wanting, or are of rarest 

 occurrence, such as Cephalopods, Pteropods, and 

 Nudibranchs. Echinoderms and Zoophytes are 

 absent. The composition of the water is inimical 

 to all these forms. The Medusae are represented 

 by shoals of the common gregarious Aurelia. 



The fishes of the Black Sea are very indicative 



