6&8 MANUAL OF THE MOLLUSCA. 
VY. ARALO-CASPIAN PROVINCE. 
The only inland salt-seas that contain peculiar shell-fish are 
the Araland Caspian. The shells chiefly consist of a remarkable 
group of Cockles which burrow in the mud (see fig. 213, p.409). 
No explorations haye been made with the dredge, but other 
species, probably still existing in these seas, have been found in 
the beds of horizontal limestone which form their banks and 
extend in all directions far over the steppes. This limestone is 
of brackish water origin, being sometimes composed of myriads 
of Cyclades, or the shells of Dreissena and Cardiwm, as in the 
islets near Astrakhan. It is believed to indicate the former 
existence of a great inland sea, of which the Aral and Caspian 
are remnants, but which was larger than the present Mediter- 
ranean at an age previous to that of the Mammoth and Siberian 
Rhinoceros. The present level of the Caspian is 83 feet below 
that of the Black Sea; that of the Aral has been stated to be 
117 feet higher than the Caspian, but is probably not very 
different; their waters are only brackish, and in some parts 
drinkable. The steppe limestone rises to a level of 200—800 
feet above the Caspian ; it spreads eastward to the mountains 
of the Hindoo Kush and Chinese Tartary, southward over 
Daghestan and the low region H. of Tiflis, and westward to the 
northern shores of the Black Sea. The extent to which it has 
been traced is represented by oblique lines on the map.* 
Some of the Caspian shells still exist in the Sea of Azof and the 
estuaries of the Dnieper and Dniester. Our information upon 
this seldom-visited region is derived from the works of Pallas, 
Eichwald,+ Krynicki,t Middendorff, and Sir Roderick Mur- 
chison. 
Aralo-Caspian Shells. 
A, Aral; C, Caspian ; B, Black Sea. 
The Species marked * are found also in the Steppe limestone. 
*Cardium edule, L.. C. (very small) B. Baltic. 
» edule, var. (rusticum, Chemn.) A. C.B. Icy Sea. 
*Didacna trigonoides, Pal. C. (Azof. M. Hommaire). 
»  Hichwaldi, Kryn. (crassa, Eich.) C.B. (Nikolaieff). 
Monodacna Caspia, Eich. C. 
», pseudo-cardium, Desh. (pontica, Eich.) B. 
Adacna leviuscula, Eich. C. 
»  vVitrea, Hich. C. A. 
* From a sketch kindly prepared by Professor Ramsay. 
{ Geogr. des Kaspischen Meeres, des Kaukasus und des Siidlichen} Russlands, Berlin, 
1830. Fauna Caspio-Caucasica, 1841. 
{ Bull, des Nat. Moscow, 1837, 
