Estuaries 27 



animals may find favorable conditions for existence. At times 

 curious associations of animals from various sources are found. 

 Crayfishes and sunfishes may consort with swimming crabs; dolphins 

 and king crabs live with river turtles and fresh-water snails. 



Working in two such widely separated estuaries as those of the 

 Ganges and the Thames, Annandale (1922) and Robson (1925) 

 are in general agreement concerning the origins and characteristics 

 of estuarine animals. Annandale points out that the fresh-water 

 fauna of the world is quite cosmopolitan. The limnaeid and vivi- 

 parid snails were already established in Cretaceous times. Animals 

 of marine origin were present in the rivers of South America, Aus- 

 tralia, China, India, and other regions. The Ganges River origi- 

 nated in the Tertiary Age. In its upper waters there are no marine 

 animals in the "highly specialized fauna," but in the lower 900 miles 

 there are four types which constitute the "relict fauna": (1) the 

 dolphin, Platanista gangeticct Lebeck, which does not leave the river, 

 and is comparable to similar fluviatile dolphins in the Amazon and 

 Yangtse rivers; (2) several species of the genus Naviculina, rather 

 primitive clams belonging to the family Solenidae; (3) Scaphula 

 celox W. H. Benson and S. deltae Blanford, clams; and Ampelisca 

 pusilla Sars, an amphipod which also occurs in the Arctic Ocean. 

 The euryhaline fauna of the Ganges delta includes: sponges, a 

 fresh- water species and a boring one on shells; coelenterates, three 

 widely distributed genera of hydroids and various species locally; 

 polychaetes; an echiuroid; many snails and clams; several species 

 of bryozoans; a king crab, Carcinoscorpius rotundicauda (Latreille) , 

 which has been seen in fresh water at Calcutta; various crustaceans, 

 including amphipods, mysidaceans, shrimps, prawns, and crabs; 

 fishes, anadromous, euryhaline, and estuarine; and a cetacean, Orca- 

 celld brevirostris Owen. There are few marine species in the river 

 proper but many in the delta. "I cannot, however, find any definite 

 dividing line between these two faunas. The relict fauna consists 



