The American Naturalist. 
ZOOLOGY. 
Excavating Habits of our Common Sea-Urchin.— The 
habit of certain species of sea-urchins of boring in solid rock is well- 
known, and has been again and again described, and made a subject 
of more or less extended study by naturalists. Although familiar with 
this work of the sea-urchins from descriptions and the study of speci- 
mens from other coasts, it has never been my good fortune to observe 
specimens of rock excavated by our species of Strongylocentrotus 
{S. drobachiensis) before the present summer. As other naturalists 
may have an interest to know of a place where this process can be 
readily seen, I have ventured to call their attention to the locality 
where it was observed. 
The eroded rocks of the shore of Grand Manan and the sunken 
ledges about it offer exceptional features for the study of the phe- 
nomenon referred to, but even here it is rare and difficult to observe, 
although for miles and miles the bottom just below low tide is paved with 
these echinoderms. There are only limited localities where the ex- 
cavating of these animals can be seen. One of the best places is on 
the Black Ledges, a few miles from Nantucket, Grand Manan. These 
ledges, wholly covered by high tide, are beaten at times by a tremen- 
dous sea, and around them course the violent tides of the Bay of 
Fundy. At low water they are bare ridges of rocks more or less cov- 
ered with kelp, their surfaces with depressions in which standing water 
remains between low and high water. The sea-urchins in these pools 
lie so closely packed together that they touch each other, forming the 
bristling carpeting of their floors. In one of these pools these echi- 
noderms have made excavations in the rock from one to three inches 
in depth, perfectly symmetrical and smooth, so close together that the 
rock has an appearance of the surface of honeycomb when the in- 
habitants of the cavities are removed. The rock in which these curious 
formations occur is a hard, gray slate, readily scratched with a knife, 
forming seams between the harder quartzite so prominent on many of 
the islands. The excavations are confined to the softer rock, and, as 
far as observed, were not seen in the quartzite. The sides of the basin 
in which they were found lie at an angle of about 80°, or almost per- 
pendicular to the floor. The ridges which separate the cavities are 
tipped by a thick, calcareous, purple alga, which, while it may increase 
the apparent depth of the cavity as measured from the rim of the same, 
is comparatively thin as respects the depth of the excavation itself. 
