866 S. W. Wallace—“Geodes” of the Keokuk Formation. 
Ido not intend by the foregoing observations to assert that 
the dorsal valve of M. Linnarsson’s A. coriacea is the dorsal 
valve of a Lingulella, or that what he sets down as ventral 
valves of A. coriacea and A. grunulata are both, or either of 
and Swedish forms compared, and which appear to me to be 
sufficiently pronounced to at least. suggest the question whether 
the Swedish species noticed. may not, as a whole or in part, be 
susceptible of a more rigorous determination. 
New York, January 22, 1878. | 
Arr. LIIL—On the.“‘Geodes” of the Keokuk Formation, and the 
_ Genus Biopalla, with some Species; by SaMUEL J. WALLACE, 
of Keokuk, Iowa. 
THE large hollow stone balls, set inside with myriads of 
brilliant crystals, which are found in the upper beds of the 
Keokuk Formation (Subcarboniferous), are well known for 
their beauty and as curiosities, under the names of Geodes, 
Niggerheads, etc. They are very plentiful, of various sizes, 
from a few lines to over two feet, where that part of the forma- 
tion is exposed in the Mississippi Valley, and in the lower 
Drift and the Alluvium derived from it. 
nessee. ‘Tio the we , 
sibly reappears in the geodes found by Professor Comstock in 
the Wind River region, Government Survey. z 
_ ‘Phe matrix bed is generally shale, varying sometimes to lime- 
stone and to porous rotten stone. The lower layers are lime- 
stone but not so pure as the next layer below, which here is the 
