194 The American Naturalist. [March, 
vary greatly in their texture, but generally are coarse and — 
irregular in grain, we find the evidence of unfavorable condi- 
tions for fossil remains in the comminuted fish bones and scales, 
except where massiveness precludes such dislocation and frac- 
ture, as with the shoulder plates of large Antiarcha ( Bothriolepis 
taylorii). 
The fresh water bivalve, Amnigenia catskillensis, is found in 
the Chemung beds, in the shaly stratum known as the Oneonta 
sandstone, at Mt. Upton, Chenango Co., N. Y., and in the Cats- 
kill beds higher up, where it is embedded in sandstone. The 
state of fossilization in these two different positions is somewhat 
contrasted. The shells of this species in the magnesian slate, 
which has been a softer and less injurious matrix than the 
sandy layers of the Catskill girt, are well preserved, indicating 
a thinner and less corrugated shell than its representatives in 
the sandstone. The shell has here been kept entire, as in a 
replacement wherein we have the pseudomorph of the two 
valves in juxtaposition, the waving and somewhat confluent 
lines of growth conspicuous, and the thin, fragile expanded 
margin preserved, whereas in the sandstone the impressions 
of the surface are less clear and distinct, and the shell-body can- 
not be removed by itself, as in the type specimen from these 
softer and less sandy beds. 
Fossils are well preserved in the slates, which, from their fine 
texture and mechanical homogeneity, take impressions easily. 
But in this group of deposits pressure has acted unfavorably in 
many instances, and the fossils have suffered distortion and 
compression. Many of our fossiliferous slates, as the Utica, 
Marcellus and Genessee, are dark-colored from carbonaceous 
admixtures, and the fossils in them partake of their color, 
which, in a measure, destroys their perfection and usefulness. 
Again, pyrite in many instances has been precipitated by of- 
ganic reduction over the surfaces of fossils in these beds, and 
the fossils then appear coated or replaced by this yellow sul- 
phide, by which they are made conspicuous upon a black back- 
ground. The interesting sponges of the Utica Slate are by this 
means beautifully retained as a network of metallic thre 
and fronds, reticulating meshes of pale gold (Cyathophycus sub- 
sphericus and C. reticulata). 
