Page:The Eurypterida of New York Volume 1.pdf/356

 of the identical structure of the compound eye of Pterygotus with that of Limulus and the determination of the endostoma.

The characteristics of this genus consist in the lack of distinct differentiation of the body into preabdomen and postabdomen; the large size of the compound eyes, their marginal position and distinct facets; the presence of a distinct epistoma, the enormous development of the preoral, chelate appendages, the slenderness and lack of spines on the four pairs of walking legs, the heart-shaped, deeply emarginate metastoma and the broad telson.

Huxley and Salter placed  in a separate subgenus, Erettopterus, on account of its bilobed telson. While some have recognized this group as a distinct genus, others have not given it even subgeneric rank. Laurie states that the species with such telsons "might fairly be separated from the rest as a subgenus, were it not that the frequent absence of the tail would make such an arrangement highly inconvenient." In our view the recognition of a natural group can not be a matter of convenience, but is of necessity, when the differences are recognized as subgeneric in rank. Erettopterus is represented in our Siluric rocks by  of the Bertie waterlime,   of the Shawangunk grit and certain telsons from the Frankfort shale.

The presence of Pterygotus in the North American Devonic has been shown by Logan, Billings and Clarke, in the Grande Greve limestone (Lower Devonic), by Dawson in the Gaspé sandstone, by Clarke in the Dalhousie formation and by Whiteaves in the Upper (?) Devonic at Campbellton. All these remains from Eastern Canada are fragmentary and only those of Campbellton have justified description.

Hall. Palaeontology of New York. 1859. 3: 418*, pl. 80A, fig. 8, 8a

. Ibid. pl. 80A, fig. 6

Hall. Ibid. p. 419*, pl. 80A, fig. 9

Description. Cephalothorax. Outline of carapace or cephalothoracic shield semielliptic. In the type of the species, the carapace of a young