THE CASPIAN SEA 573 



survived the Pontic period, and that it returned to the Caspian Sea in the 

 Akchagyl period. In the opinion of V. Kolesnikov (1941) the cyclic changes of 

 Caspian mollusc faunas in the Tertiary epoch noted by Andrussov is ex- 

 plained not by the survival of one sheltered fauna, but by consecutive migra- 

 tions of Mediterranean species into the Caspian basin. Moreover, during some 

 definite eras fresh-water forms migrated into the Caspian Sea. Thus, for 

 example, Kolesnikov thinks that the fauna of the Apsheron period, which has 

 been studed in more detail, has three origins : (7) the greatly altered remains of 

 the former Akchagyl population ; (2) fresh- water immigrants, including among 

 others Dreissensia distincta, Dr. polymorpha, Dr. caspia and Nematurella, and 

 (J) the considerably changed immigrants from the Black Sea area. Among the 

 75 Apsheron molluscs 12 are very similar to, and four are identical with, the 

 present Caspian forms. 



Fresh-water immigrants 



The more or less ancient fresh-water immigrants constitute a considerable 

 part of the fauna of the Caspian Sea. Such colonization of the Caspian Sea 

 occurred several times during its history in its periods of greatest freshening. 

 Many of the origins of the fresh-water immigrants of the Caspian Sea are lost 

 in the distant past. It has been noted, for example (B. Dybovsky, 1933, V. Bo- 

 gachev, 1932), that the very original Caspian gastropods should be considered 

 as immigrants from the fresh waters of the Pliocene ; this would explain their 

 close relationship with the Baikal molluscs. Manayunkia caspia among the 

 polychaetes, Acipenseridae and some other fish, and possibly seal, are prob- 

 ably ancient immigrants from the fresh waters of the Pliocene. For some forms, 

 the cyprinids for example, a fresh-water genesis seems more evident, and many 

 of them have apparently migrated during the late post-glacial transgression, 

 when the Caspian Sea received its last large party of fresh-water immigrants. 

 It seems certain that the Caspian Sea, and especially the Aral, were ener- 

 getically colonized by fresh-water forms, and V. Beklemishev and V. Baskina- 

 Zakolodkina (1933) have shown that this movement was not only furthered 

 by the decrease of salinity of the water. They have proved for these Seas the 

 importance of the nature of the salinity of the Caspian and Anal Seas, i.e. 

 the ratio of the magnesium and calcium ion concentrations, which brings the 

 saline waters of our southern sea-lakes close to fresh water. The ratio Mg/Ca 

 = 1-34 in the Aral Sea makes it most suitable for the existence of the fresh- 

 water crustacean Daphnia. In Caspian waters this ratio (2-5) is higher but still 

 less than that in the Black Sea (3-12); the higher survival of Daphnia in the 

 Caspian rather than in the Black Sea waters might be explained in this way. 



Mediterranean elements 



After the final separation of the Caspian and Black Seas and the linking of the 

 latter with the Mediterranean and its colonization by Mediterranean fauna, 

 some of its species penetrated into the Caspian, and even the Aral, Sea. Three 

 periods can be distinguished in the history of the Neogene colonization of the 

 Caspian Sea by Mediterranean organisms. The first and most ancient of these 

 periods apparently belongs to Khvalyn times ; it is linked with the penetration 



