146 Transactions of the Boyal Society of South Africa. 
observed that the fracture is not at the weakest part, the thin edge of the 
shell, but often passes through the thick part, at or near the hinge where 
the shell is about 3 mm. in thickness. A further point that is to be noted 
is that, while many of these shells are broken up into half a dozen or more 
pieces, others are entirely uninjured. 
An obvious and apparently sufficient explanation of this is that the 
shells have been accidentally crushed by men or animals walking along the 
sand. The same peculiarity, however, was found in unfrequented parts of 
the beach, and, moreover, it was found by experiment that the shells are so 
strong that, on treading on them heavily, they are simply pressed into the 
underlying sand. Again, other bivalve shells of a more fragile structure 
were not so damaged. Further, it was observed that fresh shells just cast 
up by the waves showed the same feature. These were still attached in pairs 
by the ligamentous hinge and contained the fresh tissue of the animal, or as 
much of it as had been left by the sea-birds, which readily devour the 
exposed body of the animal. There were two conditions observed in these 
shells. Either the two valves were intact, or one was intact, the other being 
broken into several small pieces. Seldom were both shells broken. 
The local name " Doed-Mossel " is probably due to the fact that, though 
the dead shells are found in abundance, the living animal is not often seen. 
Sometimes, however, the entire shells and the contained living animal are to 
be found. The shells are almost as firmly held together as in the living 
oyster, and cannot be opened without some instrument to rupture or cut the 
adductor muscles, and yet, in some way, the sea-bird {Lams dominicanus) 
is enabled to get at the animal soon after it is cast up by the waves. 
On one occasion a group of these birds was observed on the beach, 
feeding on the mollusc, and it was obvious that the shells had just been 
opened, the remaining part of the tissue of the animal being quite fresh. 
In some cases the two valves Avere uninjured, and in others one valve only 
was injured, having been broken in several pieces, which were found close 
by. On being disturbed the birds made off, but soon after returned. 
Observed from a distance, they were seen to pick up the shells in their 
beaks and let them drop on the wet sand from a height of about 20 or 
30 feet. It is well known that some birds have the habit of breaking open 
the hard shells of their prey by such a device, and it was at once surmised 
that this was the method of procedure in this case, though the shells were 
broken oil one side only, and then often through the thickest part of the 
shell. Moreover, this apparently did not explain the opening of the shell 
without any breaking. It was a simple matter, however, to put this to the 
test. An unopened shell containing the living animal was obtained and was 
thrown into the air to a height of 20 or 30 feet. It fell on the sharp closed 
edges of the shell and was quite uninjured. On a repetition of the experi- 
ment, however, the shell fell on its side, and this had the effect of breaking 
