The occurrence of large Crustacea other than Trilobites in the older Palæozoic rocks is so rare, that we are scarcely prepared for the simultaneous discovery of five or six new forms, of dimensions equalling those of the crabs and lobsters of the present day. Except the large Pterygotus of the Tilestone beds, and the Ceratiocaris from Westmoreland, scarcely any remains of these have yet been made out in Britain. But there have been sundry indications in the occurrence of large tail-spines and fragmentary carapaces, all of which have been most carefully treasured up to wait for further evidence, such as that now afforded by the discoveries of Mr. Slimon. His admirable collection not only fully explains the meaning of much that was fragmentary before, as in the case of the Ceratiocaris , but gives us new forms of a group of which we knew very little, except that it contained the gigantic Eurypterus of De Kay and the Pterygotus of Agassiz, and was doubtfully referable to the Pæcilopoda .