290 The American Naturalist. [April, 
of an inch in diameter, which would give nearly three million 
in a cubic inch. In the perforated walls of the favosite corals, 
where the entire skeleton of the fossil has become silicified, the 
minute pores are sometimes surrounded by little siliceous 
mamme, which seem to have gathered under the influence of 
a form of concretionary concentration. 
The peculiarities of preservation of the delicate internal ap- 
pendages of brachiopods varies extremely, and while specimens 
of the same genus or species from one locality refuse to disclose 
their loops or spiral arms to the paleontologist, those from an- 
other, even with indifferent care and easier methods, are readily 
examined. Thus, the Devonian Centronella trom England 
afforded imperfect preforations of the loop, ete., to the Rev. Mr. 
Glass, even with excessive nicety of treatment, whereas those 
from Michigan, by simply fracturing them in different direc- 
tions, established the accuracy of Prof. Winchell’s descriptions. 
In alluding to this Dr. Davidson says, “ordinarily by this 
process no certain result can be obtained, and none could be 
obtained in this manner from our Devonian specimens of the 
same genus. But in these (the American) specimens of C. Julia 
there is sufficient contrast between the color of the loop and the 
surrounding matrix to make the different parts of the loop very 
clear when revealed opaquely and by fracture. In most of the 
specimens the loop is of a rusty-brown color surrounded by a 
lighter matrix.” Again, the Athyris of our Devonian is apt to 
be filled with a dark spar, which, being impervious to light, 
yields unsatisfactory results, whereas those of the English Car- 
boniferous are filled “ with a spar beautifully transparent and 
peculiarly favorable for working.” This is by no means uni- 
versal, as Prof. Whitfield has displayed the spires of Athyris 
spirtferoides in exquisite perfection in the Hamilton slate speci- 
mens. It is singular that, as stated by the Rev. Mr. Glass, the — 
English fossil brachiopods never—or very rarely—exhibit a 
silicification of the spires in a caleareous matrix, whereas this 
frequently occurs in the United States, rendering the develop- 
ment of these delicate appendages comparatively simple, and 
incomparably beautiful. 
Few organisms are provided with a siliceous frame-work, 
and except the radiolarians and the sponges animal life has 
