THE CASPIAN SEA 



613 



and some gastropods: Micromelania elegantula, M. dimidiata, Theodoxus 

 schultzi. 



At depths of less than 50 m the Arctic species disappear, the number of 

 large crustaceans decreases considerably, while Mytilaster lineatus, Dreissena 

 caspia and Dr. rostriformis appear in large numbers, and Dr. polymorpha. 



Fig. 287. Diagram of vertical zonality of the Caspian 

 Sea (Zenkevitch, 1947). 



Didacna baeri, D.protracta and species of the genera Pontogammarus, Dikero- 

 gammarus haemobaphes and others appear in smaller numbers. On sand silt 

 above 15 m Pontogammarus maeoticus, Dikerogammarus haemobaphes, Myti- 

 laster lineatus, Theodoxus pallasi and other Gastropoda are preponderant, 

 while the Cardidae, except Didacna trigonoides, are almost absent. Ponto- 

 gammarus maeoticus lives in huge numbers on sands right at the edge of the 

 water, forming a biocoenosis very similar to that of the Sea of Azov (the zone 

 of over wash). 



