.304 
MR, W. BATESON ON SOME VARIATIONS OF CARDIUM EDULE 
The principal terraces are seven in number, but, before describing in detail the 
condition of the shells on them, it may be well to give a general account of the 
changes which were produced in correlation to the diminished size of the lake. The 
principal changes are as as follows :— 
(1.) Diminution in the Thickness of the Shells, which 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. 
(2.) The Size of the Beak is Greatly Reduced.- —In the shells of the upper terraces 
the beak encloses, so to speak, a separate chamber, while in those of the lower terraces 
it hardly forms a projection on the outside of the shell. 
(3.) The Shells become Highly Coloured. —This change and (1) occur almost 
uniformly. The shells of each terrace are very nearly alike in texture, thickness, and 
degree of coloration. 
(4.) The Grooves between the Ribs appear on the Inside of the Shell cis Ridges luith 
Reciangidar Faces. —This change first affects only the ribs behind the 8th or 10th, 
but on the lowest terraces all the ribs are so affected. 
(5.) On the lowest terrace the .shells diminish greatly in absolute size. 
(6.) The Length of the Shells in propoihion to their Breadth Increases. —I use the 
term “ length ” to mean the greatest antero-posterior dimension, 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. 
1. II. 
B 
Diagrams showing the directions in which the length and breadth of the shells are measured :— 
I. A shell from the shore of the Aral Sea. 
II. A shell from Jaksi Klich (inner deposit) L, L, length; B, B, breadth. 
It must be remembered that, though the tooth is a fixed point in the morphology ox 
the shell, there is rxo defined point on the ventral margin which can be determined in 
each shell for comparison with other shells. Hence, I am aware that the points 
selected for these measurements are arbitrary, and that they are not taken in 
absolutely homologous places in evexy shell. Nevertheless, they are very nearly so, and 
on the whole they are more satisfactory than any others. The object of these measure¬ 
ments is to obtain an arithmetical conception of the difference in the proportion 
of length to breadth which is apparent to the eye. This difference in appearance is 
