Laurie discovered the epicoxite and the second (female) form of the opercular appendage. The New York rocks furnished to Hall only fragments of the genus on which he based three species, P. cobbi, P. macrophthalmus and P. osborni. The waterlime quarries at Buffalo have since afforded somewhat more extensive series of these remains, some of the specimens excellently preserved. Grote and Pitt [1877] and Pohlman [1881–86] made use of some of these, basing thereon descriptions of six alleged species, all of which have proved to belong to two forms, Hall's P. cobbi and a new type, here recognized as P. buffaloensis (Pohlman). A fossil from the same locality described by Pohlman as a Ceratiocaris, is the telson of a gigantic Pterygotus (P. grandis). Finally the Pittsford shale, the dark shale of the Shawangunk grit, and the Frankfort shale, have furnished representatives of other species, viz, P. monroensis, P. globiceps, P. prolificus and P. nasutus, all of them as yet very incomplete. As Hall's species P. osborni has proved to be a synonym of his P. macrophthalmus, there remain out of the considerable number of proposed species of Pterygotus from the New York rocks, only eight whose differentials can now be regarded as satisfactorily determined, viz:
| P. macrophthalmus Hall | P. grandis (Pohlman) |
| P. cobbi Hall | P. monroensis Sarle |
| P. buffaloensis (Pohlman) Clarke & Ruedemann | P. prolificus nov. |
| P. globiceps nov. | P. nasutus nov. |
On this continent no Pterygoti have been found outside of New York and only two of the New York species (P. macrophthalmus and P. buffaloensis) are known in entire specimens, all the others being based on carapaces, chelae or telsons. The material of these two species, however, is so complete that it has allowed a detailed description of the forms and some additions to our knowledge of the structure of the genus. Among these is the demonstration of the identity of the structure of the "chelate antennae" of Pterygotus with the chelicerae of Eurypterus and their composition of but three segments (as already suggested by Laurie) instead of eight as formerly assumed; the demon-
