TABLE OF DISTRIBUTION. 



451 



Of the above species 96 are both northern and 

 southern, 14 are exclusively northern, and 9 exclusively 

 southern — although all the species described in this 

 volume are in one sense northern, considering Great 

 Britain to be within that line of distribution. Five 

 other species (viz. Rhynchonella psittacea, Pecten Islan- 

 dicus, Astarte crebricostata or depressa, A.borealis, and 

 Tellina calcarea) formerly existed within the area of our 

 seas, and are included in catalogues of the British mol- 

 lusca ; but they have never been discovered otherwise 

 than in a fossil state. These last are arctic, as well as 

 the Leda limatula of Say and a few more species which 

 occur in glacial deposits. The upper tertiary species 

 comprise 7 exclusively northern, and the same number 

 exclusively southern, as well as one species which is not 

 known as recent except in Shetland ; the rest are com- 

 mon to both divisions. 



