1889.] 



On some Variations of Cardium edule. 



205 



to an opportunity of investigating the question whether these diverse 

 environmental conditions produce any correlated changes in the 

 structure of the animals exposed to them. The collections made 

 with this object consist chiefly of Crustacea, of which an account 

 will appear hereafter. The shells forming the subject of the present 

 paper were collected in the district of the Aral Sea and in Egypt. 



As is well known, the Aral Sea formerly covered a larger area 

 than it does at the present time. The limits of this area have not 

 been determined, but it has been vaguely suggested that the Aral 

 and Caspian Seas together covered a large part of the steppes of 

 Western Central Asia, probably connecting with the Arctic Ocean, 

 and that the sea thus formed gradually receded until the Aral and 

 Caspian Seas alone remained in their present form. In the course 

 of my journey I visited the valley of the Irghiz and Turgai Rivers 

 and Lake Tschalkar into which they lead ; the north and north-west 

 shores of the Aral Sea ; the valley of the Shu and Tele Kul Tata into 

 which it leads ; also Lake Balkhash, and I nowhere found any direct 

 evidence which could at all support the view that the Aral Sea had 

 gradually receded from a very greatly extended area. On the 

 contrary, all trace of the previous presence of the sea disappeared 

 everywhere abruptly at a level about 15 feet higher than the present 

 level of the Aral Sea. In all places which would be covered if the 

 sea were to rise about 15 feet, shells of the cockle of the Aral Sea 

 (Cardium edule, var. rusticum) were found deposited in great quan- 

 tities, but they always ceased at a definite horizon on ascending. In 

 some places the shore of the Aral Sea is formed of vertical cliffs com- 

 posed of horizontal beds containing fossils of the age of the London 

 clay of England,* while elsewhere the beach consists of sandy flats 

 extending inland for many miles. Both in the steep places and on 

 the sands the deposits of cockles cease suddenly as described. This 

 seems to be conclusive evidence that the Aral Sea has at all events 

 not continuously receded from a very much larger area than that 

 which it at present occupies. 



On the north shore of the gulf, Sary Cheganak, a considerable 

 strip of low-lying country has been uncovered. The average width 

 of this shell-covered region is about 3 miles. The post road to Kaza- 

 linsk and Tashkend crosses it from north to south. At the post 

 station, Ak Jalpas, is a narrow, dry channel, like the bed of a river. 

 This channel passes up from the Aral Sea, and leads to two large 

 basins which were formerly connected with the sea itself, but which 

 are now dry. The southern basin is called Shumish Kul, and the 

 northern is known as Jaksi Klich. In the course of the channel by 

 which Jaksi Klich was joined to the Aral Sea is a smaller basin, 



* These fossils were kindly examined for me by Mr. T. Koberts, of St. John's 

 College, Cambridge, and Mr. Keeping. 



