his opinion, in discussing the colonization of the Black Sea by the Medi- 

 terranean fauna its most characteristic feature — the development of hydro- 

 gen sulphide in its deeper layers — should be kept in mind. Lowered salinity, 

 however, is much more important as a limiting factor. As we have seen, 

 a considerable number of animal groups living in the Mediterranean are 

 either absent from the Black Sea, or represented there by some individual 

 species. 



A. Valkanov published in 1957 a list of fauna of the Bulgarian shore of the 

 Black Sea. The list contains 343 species of four variants of Protozoa and 1,005 

 species and 23 variants of multicellular organisms. M. Bacesco's mentioning 

 of the occurrence of the polychaetes Monayunkia caspica ssp. fluviatilis 

 {danubicus), an evidently Pontic relic, in the lower reaches of river Danube is 

 most interesting. 



The process of the formation of Black Sea fauna is, possibly, incomplete 

 as yet and new forms may continue to penetrate into the Black Sea from the 

 Mediterranean. Two species of acorn barnacle (Balanus amphitrite communis 

 and B. perforatus var. angustus), discovered by G. Zevina and N. Tarasov 

 (1954) in the fouling of ships, may be included among the new immigrants 

 from the Mediterranean to the Black Sea. Continuous migration of new flora 

 and fauna forms from the Mediterranean may be seen from the example of 

 phytoplankton. Some species of Mediterranean diatoms, recent arrivals from 

 the Mediterranean {Rhizosolenia calcar-avis, Cerataulina bergoni and Lepto- 

 cylindricus danicus), have now become mass forms in the Black Sea. 



