290 A, Agassiz—Instinet? in Hermit Crabs. 
belong to the true coal-formation and cannot be separated from 
it. The conglomerate in this case can have nothing more than 
a lithological significance. 
In the South Joggins coal-field at the head of the Bay of 
Fundy, Sir Wm. Logan reports 14,570 feet of coal-measures. 
In this vast series of strata, and not very far from the middle 
of it, Dr. Dawson, in his admirable work on Acadian Geology, 
finds a group of coarse sandrocks which he denominates the 
Millstone Grit series. Here the millstone grit is not known to 
e the equivalent of the millstone grit of Great Britain, nor of 
any of our conglomerates of the United States. 
In view of this confusion, I think all will agree with me that 
the terms millstone grit and coal-measure conglomerate should 
either be used to designate a uniform horizon in the Carbonif- 
erous system or be abandoned as geological terms and retained 
only for their lithological meaning. 
I close this paper with another remark by way of inference, 
viz: that there is need of an entire revision of the classifica- 
tion of the coal-seams of the great Alleghany coal-field. It 
seems strange to begin in the middle of the vertical series und 
call a seam of coal No. 1, or letter A, and enumerate upward, 
while all the seams below are left without enumeration, But 
the data for such revision are not yet sufficient. 
Art, XL.—Instinct ? in Hermit Crabs ; by ALEXANDER 
AGASSIZ. 
captivity, commenced at once to tear out the animal, and hav- 
ing eaten him, proceeded to take its place within the shell. 
