68 MAliTUAL OF THE MOLLITSCA. 



y. Aralo- Caspian Peoyin-ce. 



The only inland salt-seas tliat contain peculiar shell-fisli are 

 the Aral and Caspian. The shells chiefly consist of a remarkable 

 group of Cockles which burrow in the mud (see fig. 213, p. 402). 

 No explorations have 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 Cydades, or the shells of Dreissena and Cardium, 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 — 300 

 feet above the Caspian ; it spreads eastward to the mountains 

 of the Hindoo Kush and Chinese Tartary, southward over 

 Daghestan and the low region E. 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,t Ejrynicki,} Middendorff, and Sir Eoderick 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). 



„ Eichwaldi, KrjTi. (crassa, Eich.) C. B. (Nikolaieff ). 

 Monodacna Caspia, Eich. C. 



„ pseudo-cardium, Desh. (pontica, Eich.) B. 

 Adacna laeviuscula, Eich. C. 

 „ vitrea, Eich. C. A. 



* From a sketch kindly prepared by Professor Ramsay. 



t G-eogr. des Kaspischen Meeres, des Kaukasusund des Siidlichen B.isslands. Berlin, 

 16<W. Fauna Caspio-Caucaaica, 1841. 

 i Bull, des Nat. Moscow, 1837. 



