Columbia subject headings:
Paleontology--Great Britain.
Paleontology--Ireland.
Paleontology--Ordovician.
Harvard subject headings:
Paleontology--Great Britain
Paleontology--Ireland.
Paleontology--Ordovician
Princeton subject headings:
Invertebrates, Fossil--Great Britain.
Paleontology--Great Britain.
Paleontology--Ordovician.