When the level of the mere, after a peculiarly dry season, is very low, is also a good time to observe the habits of many other creatures. There are always one or more crows about the neighbourhood of the lake; but at such times a dozen or so may be seen busily at work along the shore. They prey on the mussels, of which there are great numbers in the lake. Anyone passing by the water when it is so shallow can hardly fail to notice long narrow grooves in the sand of the bottom. These grooves begin near the edge--perhaps within a foot of it--and then run out into the deeper part. By following these with the eye, the mussel may often be seen in a foot or two of water--sometimes open, but more generally closed. The groove in the sand is caused by the keel of the shell as the creature moves. There are hundreds of these tracks; the majority appear to run from shallow to deep water, but there are others crossing and showing where the mussel has travelled. One may occasionally be seen in the act of moving itself, and making the groove in the sand. But they seem as a rule to move most at night, and to approach the shore closest in the darkness. In the deep water they are safe; but near the edge the crows pounce on them and may be seen peering about almost all day long. Besides those that are eaten on the shore, numbers of mussels are carried up on the rising ground where the turf is short and the earth hard. Until stepped on and broken, the two halves of the shell are usually complete, and generally still attached, showing that the crow has split the shell open skilfully. They range from two or three to nine inches in length. The largest are much less common; those of five or six inches are numerous. Some of the old-fashioned housewives use a nine-inch mussel-shell, well cleaned, as a ladle for their sugar jars. Now and then, at long intervals, an exceptionally dry season so lowers the level of the mere that all the shallower parts become land, and are even passable on foot, though in places quicksands and deep fine mud must be carefully avoided. The fish that previously could enjoy a swim of some three-quarters of a mile are then forced to retire to one deep hole only a few acres in extent. Now commences a reign of terror, of which it is difficult to convey an adequate idea.