2 THE INVERTEBRATE FAUNA 



The length of the estuary, measured from its junction 

 with the sea to the vale of Stirling, where it terminates in 

 the river, is about 56 miles. The tide, however, flows to 

 Craigforth, 25 miles above the proper estuary. Where it 

 joins the sea, its breadth, from St Abb's Head on the south 

 side to Fifeness on the north, is nearly 40 miles. As we 

 ascend it rapidly contracts, so that at Elie, 10 miles above 

 Fifeness, it is only 7 miles broad. Above this it again ex- 

 pands into an extensive basin, which at Musselburgh has a 

 breadth of about 20 miles, and then gradually narrows, until 

 at Queensferry it is only 2 miles across. Above Queensferry 

 it again expands, for a distance of about 14 miles, into a basin 

 having an average breadth of 4 miles, and terminates in the 

 river. 



The most important islands of the Firth are the May, 

 situated a little above its junction with the sea ; the Bass, 

 nearly opposite North Berwick ; Inchkeith, in the centre of 

 the greater basin ; and nearer its upper contraction the islands 

 of Inchcolm, Inch Mickery, and Cramond Island. Inch 

 Garvie lies opposite Queensferry. 



The maximum depth of the estuary at its mouth is about 

 35 fathoms ; at the mouth of the greater basin 28 fathoms. 

 From the May to Inchkeith, the greatest depth in the middle 

 of the channel is from 16 to 18 fathoms. Above Inchkeith 

 the Middle Bank stretches, separating the north channel, with 

 a depth of 16 to 25 fathoms, from the south channel or Leith 

 Koads, varying from 3 to 16 fathoms. At the upper contrac- 

 tion of the greater basin, between Inch Garvie and North 

 Queensferry, the depth increases to 37 fathoms, and above 

 this it gradually becomes shallower. The greater part of our 

 dredging has been done in the south channel, and between 

 Inchkeith and the May. 



In 1710, Sir Robert Sibbald, a learned Edinburgh physician, 

 published a list of the exsanguous (invertebrate) animals 

 which he supposed were common to the Firths of Forth and 

 Tay.* He divides them into four classes : (1.) the Holies, 

 among which are to be found Cephalopods, Asterids, and 

 Medusae ; (2.) the Crustrate, comprehending Crustacea and 

 * " History of Fife and Kinross," p. 53. Edin., 1710. 



