16 The American Naturalist. [January, 
vating in a ring-shaped depression (Fig. 4, A. and B.) enclosing 
the central area. The animals are assisted in their work by the 
presence of small stones (c), which have been washed in by the 
waves. An interesting observation in relation to the sea-urchins 
is that on the inner surface of the pot-hole there are many small 
excavations worn by the sea-urchins (e). Each sea-urchin is 
lodged in a cavity which it has worn out. Moreover, there are 
also similar holes (Fig. 2) excavated by the sea-urchins with the 
animals still in them on the central style. The Echinoids occu- 
pied their supplementary excavations when they were seen by 
Prof. Marcou, and the cavities are so shallow that a small 
part of the body of the sea-urchin projects beyond the level of 
the rim of the hole. The position is certainly exceptional, but 
not less remarkable than the fact that we have a pot-hole with a 
central columella.” Prof. Marcou also observed “ pot-holes,” and 
the surfaces of the enclosed stone (s) were tenanted with sea- 
urchins which were found in slight depressions. The size of the 
enclosed stones would seem to imply that they are due toa 
breaking away at the base of the central style of the first kind, 
and not that they were the active agents in wearing down the in- 
ner surface of the pot-hole. 
The first type (Fig. 1) of pot-holes with sea-urchins, which 
were observed by Prof. Marcou, may be nothing more or less than 
ordinary “ pot-holes ” in which the wearing stone is found at the 
bottom. They are interesting as possessing supplementary 
cavities worn in the sides by the sea-urchins. This type may 
have been either derived from the third (Fig. 3), that with a 
larger enclosed stone, which itself came from the stylated (Fig. 
2), or it may have been developed, as in the case of ordinary pot- 
holes which do not have the stylated axis, by simple grinding 
resulting from the movement of the stone. It can readily be 
seen that the third instance (Fig. 3) mentioned by him is derived 
from the second (Fig. 2), and that the second may have been de- 
rived from the fourth. The first, however, may have been formed 
as all simple pot-holes, by the rocks or pebbles within it, as ordi- 
28 It certainly cannot EC i a ca gen of this 
kind are formed in the same way as ordinary kin 
