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A PRELIMINARY NOTE ON THE MANNER IN WHICH 

 THE OYSTERCATCHER (H.EMATOPUS OSTRA- 

 LEGUS) ATTACKS THE PURPLE-SHELL (PUR- 

 PURA LAPILLUS). 



By J. M. Dewar. 



The Oystercatcher feeds upon the contents of the Purple- 

 Shell to a limited extent it is true, yet sufficiently to justify its 

 inclusion among the forces which control the numbers of this 

 predatory mollusc. The opened shells which I have collected 

 vary from three-quarters of an inch to one inch in extreme 

 length from apex to base. The shells are detached from the 

 rock with apparent ease. The Oystercatcher passes or forces 

 its bill well under the shell, and by a quick lateral movement of 

 the head tips the shell over so that the aperture looks upwards. 

 In this position it is seized crosswise within the tips of the man- 

 dibles, and carried to a suitable place ; it may be a little crack 

 in which the shell is laid lengthwise, or a slight hollow in the 

 rock, or very often a patch of firm sand. These advantages are 

 optional, and room to work in seems to be the main object. 

 Having set down the shell the Oystercatcher pushes its bill over 

 the outer lip in a downward direction, introducing into the aper- 

 ture the upper mandible alone. Through the point of the latter 

 it administers a number of hammer-like blows, or, resting the 

 point on the interior, a series of powerful thrusts, or pressing 

 firmly and continuously it moves its head slowly from side to 

 side, as if imparting a rolling motion to the shell in the direction 

 of its long axis. If the bird is successful, a fragment is displaced 

 from the under side of the shell. It is disc- shaped, and about a 

 quarter of an inch in diameter. The edge may be straight, 

 more usually it is bevelled inwards. The circular opening to 

 which the disc corresponds is divided into two parts by the 

 margin of the operculum, thus proving that the point of the 

 upper mandible is pushed into the angular recess between the 

 operculum and the inner wall. This marks the end of the first 

 stage, and frequently it is never reached, many of the shells 

 being strong enough to resist the efforts of the Oystercatcher. 



