10 The American Naturalist. [January, 
smooth character of the floor of many of the sea-urchin hollows. 
One can readily imagine how in the beginning of the growth 
of the cavity, in order to cling more tenaciously to the rock, the 
sea-urchin seeks out a small natural depression in which it 
anchors itself by its suckers for protection from the surf. Con- 
stant movement of its body, due to the waves, and consequent 
attrition of the spines and teeth, caused by the movement of the 
water, deepens this slight depression until it reaches a consider- 
able size, which is enlarged by every motion of the animal. To 
this increase in size voluntary movements of teeth and spines 
also contribute.” 
The accompanying figure taken from a photograph by Mr. 
Northrop, of the School of Mines, New York, represents a 
number of these sea-urchins as they naturally lie in their ex- 
cavations, and also several cavities, formerly occupied, from 
which the inhabitants have been removed. Although these 
holes are not so deep and therefore not so conspicuous as can be 
seen in rocks from some other coasts which might be introduced 
in illustration, they are the only ones which I have ever seen 
from our vicinity which exhibit this phenomenon” 
I am tempted to return to an aspect of the subject to which I 
have already alluded. Notwithstanding the large numbers of 
sea-urchins found at Grand Manan, the places where the excava- 
ting habit can be studied are very few in number and very local. 
I know indeed of no other locality except on the Black Ledges 
where there are excavations of this kind, and yet there are many 
places where the conditions are identical, and where the animals 
are equally abundant. It may be concluded from these facts that 
there must be certain peculiarities of environment especially 
adapted to these animals to present favorable conditions in indi- 
viduals for this habit. It would seem that a mechanical explan- 
ation that the depressions were in a way due to forces analogous 
to the action of those at work in the excavation of “ pot-holes ” 
*1 James Salter. On the Structure and Growth of the Teeth of Echinus. Quart. 
Journ, . Sc., 1861, p. 216. 
2 The fact that specimens illustrating the excavating power of the sea-urchins can 
. oast is an interesting one as affording an opportunity to study the 
method by which the sea-urchins accomplish their work. 
