490 =a. Trowbridge—Method of freeing a Magnetic Bar 
looks like de Verneuil’s figures; but, when we remember that 
his species came from the Permian rocks of Russia, and that 
his typical specimens consist merely of imperfect impressions in 
a mass of rock, it must be admitted that we have very unsatis- 
factory means of comparison, and that the probabilities are 
much against the conclusion that this American Coal-measure 
shell is really the same as the Russian species. 
It is very similar to Nuculana bellistriata (= Leda 
bellistriata Stevens), originally described from our western Coal- 
measures, and very common at that horizon in Illinois, to 
which species I referred it as a variety (N. bellistriata, var. 
attenuata), in Hayden’s Nebraska Geological Report of 1872, 
page 206. It seems, however, both in Illinois and Nebraska, 
to be always decidedly smaller, and more attenuated at the 
sche extremity, than the form generally referred to Dr. 
tevens’ shell, and [ am now inclined to regard it as a distinct, 
more delicate species. Its smaller size is also quite as strong 
an objection to the supposition that it may be identical with 
de Verneuil’s species, 
[To be continued. } 
Art. XLVIL—Brief Contributions from the Physical Laboratory 
of Harvard College. No. V.—On a method of freeing a Mag- 
netic Bar from the influence of the Earth's Magnetism ; by JOHN 
TROWBRIDGE. 
