THE EUROPEAN SEAS. 



233 



rocks. Sands favour the genera Panopcea, My a, the 

 Solen tribe, Donax, Tellina, Mactra, Tapes, Venus, 

 &c. This, however, depends in a great measure 

 on the description of sand. A very large propor- 

 tion of the bivalved Testacea, of all seas, occur over 

 a sea-bed of muddy sand; but there is a zone of 

 clean sand in advance of most lines of coast which 

 comes within the range of the tidal and wave dis- 

 turbance of the water, where deposits are being 

 formed, which, after a while, are broken up again, 

 and which may be called the drift-sand zone. This 

 is wholly unfitted for marine life, and the only 

 organic forms it ever contains consist in the frag- 

 mentary shells and tests of other zones. I have 

 dredged along a band of this kind for thirty miles 

 on our own coast without finding a single living 

 form. 



In muddy lagoons Scrobicularia is abundant. 

 Never a and Isocardia prefer deep-sea mud. 



On our coasts lines of shingle, passing down into 

 running sands, are not prolific in animal life. More 

 in the offing, and in situations where scollop banks 

 have established themselves, there is usually found 

 a rich and varied harvest of MoUusks and Ophiurae. 



Eocks rising somewhat abruptly out of deep 

 water cannot be dredged, nor indeed can a rocky 

 sea-bed, but the multitude of dead shells met with 

 in the vicinity of such submarine conditions shows 

 how favourable they are for supporting life. Gaste- 

 ropods abound in such situations, and I have known 



