442 



BIOLOGY OF THE SEAS OF THE U.S.S.R, 



farther and deeper out into the Sea'. Shell gravel usually occupies separate, 

 isolated areas on the shores of the Black Sea, not forming a continuous 

 band. Well up inside the bays it usually rises to a depth of a few metres, 

 while in the open sea it may be as deep as 55 to 65 m. Shell gravel consists 

 mainly of the molluscs which inhabit the sand lying above it ; it contains an 

 admixture of oysters, mussels and some other forms. An oyster form called 

 Ostrea sublame/losa (cliff oyster) lives on the cliffs, and the O. taurica variety 

 (bank oyster) lives on the oyster-beds (Fig. 212). 



\ SAND 



MUSSEL OOZE 



PHASEOLIN OOZE 



Fig. 212. Oyster bank off eastern coast of Black Sea 



(Nikitin, 1934). A in plan. В cross-section; 1 Sand with 



Venus ; 2 Oyster bank ; 3 Phaseolin ooze. 



Thus the oyster-bed is a variety of shell gravel. Shell gravel is separated 

 from the zone of sand lying above it by imperceptible stages, while the 

 biocoenosis of deeper-lying mussel- and phaseolin-oozes is separated from the 

 shell gravel much more sharply. 



Apart from oyster the following molluscs are components of this biocoeno- 

 sis : Mytilus galloprovincialis, Pecten ponticus, Tapes rugatus, Venus gallina, 

 Cardium edule, С exiguum, Modiola adriatica, Merethrix rudis, Nassa reti- 

 culata, Gouldia minima and others ; it contains also the crustaceans Porcellana, 

 Athanas, Portunus arcuatus and P. marmoreus; the hermit crab Diogenes 

 varians ; Balanus improvisus ; a mass of polychaetes, sponges (especially the 

 small boring sponge Cliona stationis) and hydroids. The shell-gravel biocoe- 

 nosis is the Black Sea group which is richest in its composition. 



