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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 
30 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 
Roads, 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 Molles, 
among which are to be found Cephalopods, Asterids, and 
Medusee ; (2.) the Crustrate, comprehending Crustacea and 
* ** History of Fife and Kinross,” p. 58. Edin., 1710. 
