Columbia subject headings:
Harvard subject headings:
Paleontology--England.
Paleontology--Silurian
Paleontology--Wales.
Trilobites--England.
Trilobites--Wales.
Princeton subject headings:
Paleontology--England.
Paleontology--Silurian.
Paleontology--Wales.
Trilobites--England.
Trilobites--Wales.