Though a great deal has been written on the Eurypteridæ, and many points of their anatomy elucidated in the brilliant memoirs of Huxley and Salter, Hall, Woodward, Schmidt, &c, nevertheless many points of morphological importance remain obscure. This is perhaps to be attributed to the fact that nearly all the writers on this group have treated them rather from the systematic than the morphological standpoint. In dealing with remains so fragmentary and obscure as the majority of these fossils are, the value of some theory as to their relations among recent forms is enormous, both as suggesting points to be looked for and aiding in the interpretation of structures observed. The greater part of the work on this group was done before the arachnid relationship of Limulus was fully appreciated, and it is in the light of a possible relationship to this form, and also to the lower orders of terrestrial Arachnida, that it seemed to me to be worth while to revise the anatomy of the group. It has been necessary to include a certain amount of what is already well known in the description of the different genera, and I have taken special care to confirm, as far as possible, points which seemed to me to rest on insufficient grounds.