206 



Mr. W. Bateson. 



[June 6, 



Jaman Klicli. When the level of the Aral Sea fell, each of these 

 three basins became isolated as a separate salt lake containing a 

 sample of the fauna of the Aral Sea. All three lakes subsequently 

 dried up, and during this process the water must have become Salter 

 and Salter until only a bed of salt remained. 



Shumish Kul is about 8 miles long. The western shore is bounded 

 by high limestone hills, on tbe foot of which definite terraces are 

 marked by the action of the water as the level of the lake gradually 

 sank during the process of drying up. On these terraces are great 

 numbers of shells of the cockle of the Aral Sea (Gardium edule,var. 

 rusticum), being for the most part paired shells with their ligaments 

 preserved, fixed on the oral faces in the crust of oxide of iron and 

 mud, which was formed when the waters of the lake covered them. 

 A series of specimens, therefore, taken from these terraces gives 

 examples of the shells as they were at each stage in the progressive 

 desiccation of the lake, and shows the changes which they underwent 

 as the lake became salter and Salter. The principal terraces are 

 seven in number, and the total difference in level between the top 

 and the bottom is about 60 feet. 



On comparing the shells found on successive terraces from above 

 downwards, the chief variations noticeable in them are as follows : — 



(1.) Diminution in the Thickness of the Shells. — This is first apparent 

 in the shells of the third terrace. It proceeds to such an extent that 

 the shells of the lowest terrace are almost horny and semi-transparent. 

 The change in the thickness of the shells is most clearly brought out 

 by the table giving the comparative weights of the shells from 

 different terraces. 



(2.) The Size of the Beak is greatly reduced. 



(3.) The Shells become highly coloured. — This change and (1) occur 

 almost uniformly. The shells of each terrace vary very little among 

 themselves in texture, thickness, and colour. 



(4.) The Grooves between the Bibs appear on the inside of the Shell as 

 Ridges with rectangular Faces. — This change first affects only the ribs 

 behind the 8th and 10th, but on the lowest terraces all the ribs are so 

 affected. 



(5.) On the lowest Terrace the Shells diminished greatly in absolute 

 Size. 



(6.) The Length of Shells in Proportion to their Breadth increases. — 

 [ use the term " length " to mean the greatest anteroposterior dimen- 

 sion, and the term " breadth " to mean the dorso- ventral measurement 

 at right angles to the length, passing in right valves across the point 

 of the posterior tooth, and in left valves across the depression into 

 which the posterior tooth of the right valve fits. I have found by 

 careful testing that these measurements can be relied on to 0*5 mm. 



In the course of these measurements it appeared that — 



