These are the plate-like swimmerets and opercula of Gigantostraca and Limulus among Arachnids and of Isopod Crustaceans.
In external form Trilobites are not unlike Isopod Crustaceans, especially the terrestrial species commonly called "woodlice"; and until the nature of their appendages was known, it was thought by some authorities that the two groups might be related.
At Cambridge he obtained fossil shells from the Pleistocene deposit at Barnwell; in the Vale of Wardour he discovered in Purbeck Beds the isopod named by Milne-Edwards Archaeoniscus Brodiei; in Buckinghamshire he described the outliers of Purbeck and Portland Beds; and in the Vale of Gloucester the Lias and Oolites claimed his attention.
