110 THE ZOOLOGIST. 



The rolling motion to which allusion has been made can be 

 demonstrated experimentally when the bill or other instrument 

 is introduced into the aperture as near the apex of the shell as 

 possible. The terminal portion rests in a vertical furrow, and 

 when the upper end is oscillated in the direction of the long axis 

 of the shell the latter rocks to and fro ; as it rises on the apex 

 the part directly under the bill, being unsupported, is driven out 

 with moderate force in the typical form of a disc. 



At the second stage the shell is turned over so that the 

 normal aperture looks downwards or to one side. The Oyster- 

 catcher picks up and drops the shell to make it roll until it 

 rests in the desired position. Usually one rolling is sufficient, 

 but it may have to be repeated once or twice. From its form 

 the shell can come to rest in one or other of two approximate 

 positions — with the abnormal opening looking upwards, or with 

 the aperture uppermost when the abnormal opening looks to one 

 particular side. It cannot face the other side, because the shell 

 will at once roll into its original position. When the abnormal 

 opening looks upwards — perhaps the more common result — the 

 Oystercatcher pushes its bill into the body whorl towards the 

 ground and the apex of the shell, and then lowers its head in 

 one swift, powerful movement. The outer lip appears to form 

 the fulcrum of the lever. By its inclination away from the axis 

 of the shell and by its sharp edge it resists the tendency of the 

 shell to revolve. This resistance, however, must be increased 

 by the line of leverage being as much as possible in the long 

 axis of the shell. The methods applicable at the first stage may 

 be used instead of that just described. When the abnormal 

 opening looks to one side the Oystercatcher may employ that 

 method, but a considerable part of the leverage will be wasted 

 before the sharp edge of the outer lip bites the ground, and 

 generally the methods of the first stage are adopted, the upper 

 mandible alone passing through the abnormal opening, and 

 travelling as much as possible towards the ground and the apex 

 of the shell. Probably there is here an inclination towards the 

 continuous pressure and rocking manoeuvre. The result is 

 strikingly uniform. A second piece of shell is driven out on the 

 side of the first abnormal opening nearer the apex and further 

 from the aperture. Nearly twice the diameter of the first disc, 



