18. The American Naturalist. [January, 
dropping off, fall down between the sea-urchins, and by their 
grinding action serve to enlarge the depression. The stones, 
however, are too large to be placed there by the sea-urchins, and 
it is probable that they were simply caught in the depression 
after having been thrown there by the waves. 
Prof. Marcou has furnished the following directions for those 
who would examine the interesting pot-holes which he has dis- 
covered at Biarritz : 
South of “le port Vieux” there push out to the west into the 
sea at least three peninsulas, separated by parallel bays. The 
road to the Côte de Basques runs at the base of these peninsulas, 
on the eastern edge. The pot-holes were observed at the western 
end of the most western of these peninsulas. The locality is 
rich in fossil echinoidea, serpulz, and other animals. 
The question of why and how the sea-urchins bore in the rock 
seem to have been answered in as many ways as the number of 
observers. The sea-urchin excavations at Grand Manan have cer- 
tain resemblances to and differ somewhat from those recorded by 
others. My interpretation of their cause, as derived from their 
study at this locality, also differs somewhat from those of other 
observers. As to the question why the sea-urchins make these 
hollows there seems at least two answers. Their main object is 
probably for protection from the waves, as has already been 
shown by several naturalists. At the same time that the depres- 
sions serve for protection, they would form receptacles for water, 
which would be of great use to the animal between tides. 
2 Ordinarily not much water would be retained in that way, but 
in the case of the pot-holes at Biarritz it would be considerable 
in amount, and natural aquaria with a continual supply of pure 
water would be at hand. Prof. Marcou has aptly compared the 
situation of these colonies of sea-urchins on the walls of a pot-hole 
to a hotel with guests. The simile, though fanciful, may not be 
far from the truth. It is a significant fact that sea-urchin excava- 
tions where the tides are small, as in the Mediterranean, are also 
not very large. 
30 Marcel de Serres (Sur l'action perforante de l’Echinus lividus. Comp. Rend., Vol. 
XVIII., pp. 405, 406) tries to explain the rarity of sea-urchins’ excavations on the coast 
i r by the absence of the ebb and flow of the tide. 
