Columbia subject headings:
Geology, Stratigraphic--Cambrian.
Geology, Stratigraphic--Ordovician.
Paleontology--Cambrian.
Paleontology--Ordovician.
Harvard subject headings:
Geology, Stratigraphic--Cambrian
Geology, Stratigraphic--Ordovician
Paleontology--Cambrian
Paleontology--Ordovician
Princeton subject headings:
Geology, Stratigraphic--Cambrian.
Geology, Stratigraphic--Ordovician.
Paleontology--Cambrian.
Paleontology--Ordovician.