of the Fishery Board for Scotland. 
213 
the two shells looking upwards and forwards. Their siphonal end — 
with the inhalent and exhalent openings — is therefore generally directed 
more or less upwards. On being touched they withdraw their siphons 
within the valves of the shell, which firmly interlock with each other, 
the prominent ridges of one edge of the shell fitting into and inter- 
locking in the corresponding depressions in the edge of the other. The 
siphon tubes are short and conical, and diverge somewhat towards their 
free ends. At the opposite end from the siphons the foot is protruded, 
It is elbow-shaped, the distal end being pointed, and of a yellowish 
brown colour. Like the other Lamellibranchs which have the power of 
burrowing in sand or mud, by the alternate lengthening and thinning 
of the foot, on the one hand, and by the thickening and contracting from 
the point backward, on the other, the cockle is enabled gradually to 
work its way forward in the sand. The reverse or ' shoving ' process 
permits its movement backwards. 
While this is the method of movement of the adult cockle, the embryo 
leads a free pelagic life, swimming by the aid of the embryonic organs, 
which afterwards become aborted. 
The thickness of the shell is extremely variable. In hard sandy 
beaches the shell is thick and globular in form, as it also is in exposed 
stretches of the coast; but where the ground is soft and more muddy the 
shell is slightly elongated posteriorly, and also thinner than the globular 
form. The thickness of shell and shape depend on the nature of the 
deposit in which the animal lives and on the forces that strike against the 
sandy beaches. 
In large and globular-shaped specimens the beaks of the valves are so 
prominent, that by applying them back to back with the hinges at an angle 
of 90 degrees, one can open them by a corkscrew-like motion of the one on 
the other. In many cases, however, the shells are so soft that the beaks 
or umbones are worn away without the valves being divaricated. This 
softness is due to the composition of the shell, in which { Dr Phipson has 
' shown that they contain more than 90 per cent, of pure carbonate of lime.'* 
The great quantity of carbonate of lime in the inorganic constituents of 
the shells renders these admirable as a source of lime supply, which is 
easily obtained by calcining them. Tho Old Statistical Account says, 'the 
* shell of the cockle makes the whitest, if not the strongest lime.' 
3. The Barra Cockle Beds. 
At the northern end of the Island of Barra, and facing the eastward, 
are the sand banks where the cockle is found. The sands are divided 
into three distinct portions — Traigh Mhor (in the Admiralty chart spelt 
Trigh Vore) being the most southerly, Kilbar Strand or Traigh Cille 
Bharra next, and Traigh Sghuir'aval (spelt in the Admiralty chart Trigh 
Scurrival) lying at the extreme south end of the peninsula of Eoligary. 
Traigh Mhor, the most important of the three, is divided from Kilbar 
Strand by the Island of Oronsay, and the latter from Traigh Sghuir'aval 
by the point that juts out nearly opposite Eoligary House towards the 
island of Fuday. Although there are great banks of sand on the west 
side of Barra, especially opposite Borve, and south and north of Grean 
Head, yet the exposure of these to the heavy seas of the Western Ocean 
prevents the cockle obtaining a foothold there. In the case of the sand 
banks of Traigh Mhor, Kilbar Strand, and Traigh Sghuir'aval, as these lie 
on the east side of Eoligary peninsula, they form good ground for cockles. 
* Jeffrey, loc. ciL, p. 291. 
