196 The American Naturalist. [March, 
a sesquioxide, and so the contained remains of animals are 
changed from a blueish gray to a ferruginous yellow, and be- 
come more conspicuous. Fossil shells possessing a smooth, 
compact surface seem to have often escaped disintegration, 
while the surrounding matrix was removed, because they af- 
forded no absorptive surfaces for the retention of the dissolving 
infiltrations of acid waters. It must also be borne in mind that 
the changes which limestone beds undergo from secondary 
erystallization, produced by the geognostic conditions of pres- 
sure, heating, etc., affect all their contents, and develop a crys- 
talline structure in which the fossils become scarcely recog- 
nizable. 
Invertebrate fossils occur as moulds, casts, or entire shells 
and bodies, according as they have left their impressions in the 
soft sediments amidst which they lay, or have been filled by 
the penetration of the ocean mud, so as to have produced a 
complete cast of their entire interior shape and markings; OF, 
lastly, as they have been petrified throughout, and remain as 
they were deposited, retaining their exterior shell or integu- 
ments, and presenting when extracted a stone counterfeit of 
the original organism. Of course, for the most part, in such a 
process of petrifaction all interior structure is destroyed. But 
this is not universally the case, and many important interior 
appendages are sometimes preserved in exquisite perfection, 0T, 
at least, so far preserved as to afford instruction to the paleon- 
tologist. This preservation is determined by the nature and 
substance of these parts, and depends also upon the mineral 
conditions existing during the fossilization of the animal. 
Thus, the brachial appendages of Orthis, Strophomena and Pro- 
ductus are of such a soft and fleshy texture as to be unable 
to resist change long enough for their conversion into 4 
mineral framework, although their impressions are found, 
whereas with Spirifera, Terebratula and Atrypa these delicate 
spirals and loops, upon which the breathing arms are support 
in the chamber cavity of the shell, are preserved in a siliceous 
reproduction. Again, a sandy deposit will seldom permit the 
perfect preservation of fragile portions of the animal, nor 10- 
duce that gradual replacement of its calcareous structure by 
