metastoma or post-oral plate found both in the same association and attached is of similar form, with a proportionally greater width than in the ordinary forms of Eurypterus, and approaches more nearly to the form of the same appendage in Pterygotus. The single dactylus at the extremity of the palpi corresponds with known forms of Eurypterus described by Mr. Henry Woodward, and may also be compared with the corresponding part of Pterygotus. The spiniform extensions at the post-lateral extremities of the segments of the body and thorax are but a more extreme development of a feature which is common to all true Eurypterides, and can scarcely be considered of generic importance.
In the elongated joints of the swimming foot and their serrated margins the Pennsylvania forms resemble Dolichopterus, and may be referred to that sub-genus with as much propriety as to any other sub-generic form. In this respect the Eurypterus (Anthraconectes) Mazonensis is still more similar to Dolichopterus in its simple elongate median appendage of the thoracic plate. The two small accessory lateral plates of the median appendage of that species have not been detected in E. Mansfieldi, and their presence in the former species may perhaps, if verified, be considered as of sufficient importance for the separation of that form as a sub-genus. Finally, the forms are not as far removed from the typical species of Eurypterus, in any of their characters, as are several of those which Mr. Woodward has described from the Upper Ludlow rocks of England, and which he does not hesitate to place under the genus Eurypterus.
The Pennsylvania forms are quite unlike the European carboniferous species in their general aspect and proportions, as well as in the details of parts of their organization, and in these respects more nearly resemble the typical forms or the genus.
In addition to the two very well-marked forms described, there are, in the collections examined, several fragments of other crustaceans of this family which cannot be satisfactorily referred to genus and species. Two of these are figured on Plate IV, Figs. 9, 10. The specimen, Fig. 9, is an ectognath belonging to a large form of which we have also
