Sphenophyllum is a commonly found fossil in the shales of the Llewellyn formation at St. Clair, PA.
Sphenophyllum is part of the Horsetail family and is generally thought to be a creeping or climbing plant that may also have been an understory shrub in the lycopsid forests (Lepidodendron, Sigillaria). As is the case with many fossilized plants from the Pennsylvanian, the generic name Sphenophyllum originally included only impressions/compressions of certain leaves, but has come to include several types of leaves, stems and roots. Early paleontologists were looking at fragments of the plant without a good idea of how everything fit together. As a result they gave genera level names to many different parts of the same plant. Today we know more about what the plants looked like thanks to some well preserved fossils and much detective work but we still retain some of the various names given to the parts.
I usually find sections of the stem with some leaves attached as whorls around segments.
I think this piece is a segment from a branch with some of the thinner, more needle like leaves.
This is a single whorl of leaves that separated from the branch and looks like a flower, however flowers would not develop for another 100 million years. The preservation is fairly good and you can see the veins in the leaves.
For a great illustration of a Sphenophyllum reconstruction, see this post from the Fossil Forum.
This UMCP of Berkely page has a nice page that discusses the origins of Sphenophyllum.