APPARENTLY CORRELATED TO THE CONDiTTONS OF LIFE. 
311 
The average ratio of length to breadth in 30 shells of the Aral Sea varying in length 
between 22 and 18'5 mm. is 1 :0'7G1. 
The beaks in every case are large and well developed. 
On the west shore of the Sary Cheganak, near Alta Kuduk, is a small dry lagoon, 
which had once communicated with the Aral Sea. It was about half a mile wide 
and had been about 2 to 3 feet deep. In it were many Cockle shells; nearly all of 
these were “ great shells,” the remainder being shells of the ordinary Aral type. 
This completes the description of the Cockles of the district of the Aral Sea. It 
has been shown that in each locality a particular type prevails, w'hich varies hardly at 
all as regards texture and colour, and that, though the individuals of each type vary 
considerably in shape, yet that there is a distinct preponderance of long shells among 
those wdrich have been exposed to the conditions incidental to the drying up of the 
lakes in which they were living; and that, in the case of each of three lakes, the 
changes undergone by the shells have been similai', though difierent in degree. 
I will now describe the shells found in the lagoons near Alexandria, and then 
compare them with those of the Aral Sea district. 
The Cockles of Lake Mareotis and Lake Ahu Kir. 
At the present time (1888)'“ Lake Abu Kir is a shallow salt lake, having an area of 
about 20 square miles and a depth of about 1 to 2 feet at most. In April, 1888, its 
specific gravity was I’Oo. No living shells were found in it but its shores were 
covered with great quantities of uniformly small, thin, highly coloured shells (see 
Plate 26, fig. 10). These shells are elongated in the same way as those of Jaman 
Klich, which they closely resemble. 
The average ratio of length to breadth in 30 of these shells varying in length 
from 24 mm. to 19’5 mm. is 1 : 0‘738. (For average weight see Table of Weights). 
These shells are plainly those of the Cockles which last lived in the lagoon of Abu 
Kir, and it may be supposed that they lived in it under conditions not very different 
from those now prevailing. It is difficult to assign with certainty any cause for their 
extinction, but this may perhaps have been due to an unusually dry season following 
on a low Nile. 
The lagoons of Abu Kir and of Mareotis are separated from the sea by a narrow 
bank, partly of limestone and partly of sand ; and from the presence of marine shells 
in the lagoons it is clear that they formerly communicated wdth the sea. The Cockles, 
therefore, of these lagoons are the descendants of those of the Mediterranean. 
There is some reason for supposing that they passed through another condition 
between that of the Mediterranean type and that found on the shores at Abu Kir; 
for at Mandara and at other points on the shore of the Lake Abu Kir, where 
cuttings have been made, deposits of great quantities of shells almost invariably occur 
at a varying depth below the surface. 
* Abu Kir was pumped out in May, 1888. 
