1890.] Excavations by Sea-Urchins. 5 
the rim which forms a dividing edge between two contiguous ex- 
cavations is often covered by an incrustation of genera of 
calcareous alge. 
Prof. W. G. Farlow, of Harvard University, has kindly identified 
some of the corallines from specimens of the rock submitted to 
him. He finds upon them, Lithothamnion polymorphum (L.) 
Aresch., and Melobesia lenormandi Aresch. A third kind with 
papilla, which he interprets as the beginning of branches, he is 
inclined to regard a young stage of Lzthothamnion fasciculatum. 
He writes me that “the corallines of Northern Europe have 
been more studied since date of his Marine Alge of New 
England, and there may be a question whether the nomenclature 
of our Eastport species should not be modified to adapt itself to 
recent work in Artic corallines.” The most abundant coralline 
in the neighborhood of the excavation is the first mentioned. 
From the sections which I have made of alga, and rock, and a 
minute examination of the line of contact, I cannot see that the 
_ former has in anyway modified the surface of the latter, a result 
practically the same as that arrived at by John in his studies. 
While it is possible that this deposit may in some instances 
augment the depth of the depression, the amount of such increase 
is not large, for the alga is generally limited to the top of the 
bounding rim of the depression, and the walls of its 
deepest part are filed perfectly smooth, while the sides of the 
depression and more especially the bounding rim are sometimes 
covered with and augmented by this encrusting coralline. The 
surface of the hole as above remarked had no coralline, and gen- 
erally the rock appeared to be freshly cut. 
Hesse records an observation of a sea-urchin boring in oyster 
shells, and regards this habit as a means of getting food. While 
it is not impossible that a modicum of nourishment may be 
obtained in this case by the sea-urchin, it seems to the author im- 
probable that the amount thus obtained can be any considerable 
quantity. I have learned from good authority (Mr. Cheney) that 
the spiles of weirs at Grand Manan are sometimes excavated by 
sea-urchins so that the bark of the birch posts is worn through 
and even depressions made in the wood tissue. In this instance 
