NOTES ON THE OYSTERCATCHER. 209 
may have the valves separated slightly when the Mussels are in 
position, and the opening up completed at leisure after detach- 
ment. The reason is to be found in the characters of the two 
borders. Along the dorsal border the valves meet at an acute 
angle and rest insecurely on the ground, while on the other 
side they meet at an open angle, and are flat-bottomed like a 
barge. 
The Oystercatcher empties a shell at the bottom of a Ce 
hole in the sand as easily as one on the open ground. 
After the shell has been opened the separation is effected by 
introducing the upper mandible within the shell, and by gripping 
a valve—usually the damaged one—between the mandibles, a 
few vigorous shakes and a pull in the upward direction being 
sufficient to detach the shell. 
Ill. Mussels opened through the posterior ends. 
Forming about thirteen per cent. of the shell remains, the 
valves of these Mussels are never fractured, and at most show 
some comminution of the thin posterior edges. The fragments 
remain attached to each other, and are not driven inside the 
shell. 
It is, therefore, likely that the valves are separated to some 
extent before the bill is introduced. This can be the only route 
to the interior of the buried shells, the long axes of which are 
vertical, but in the case of Mussels placed horizontally on the 
banks it is not easy to understand why the posterior end of each 
should be chosen. 
The method is simple enough. The point of the bill is 
inserted quickly between the valves and pushed home by a 
number of forcible jerks. Vigorous shaking of the bill sidewise 
follows, and is sufficient to open the shell. When the Mussel 
lies horizontally the Oystercatcher approaches from behind with 
its head lowered nearly to the ground, and the point of the 
bill directed forwards. 
A feature which the shell opened at the posterior end exhibits 
more frequently, and to a greater degree than the shell opened 
by any of the other routes, is a partial rotation of one valve on 
the other, about a point situated near the middle of the ligament. 
