Page:The Eurypterida of New York Volume 1.pdf/165

 especially in the clawlike development of the ninth or terminal segment of the swimming leg, and it is made the type of the subgenus Onychopterus. has a long carapace with eyes well forward, characters that are also present in the young of. , from the Guelph formation, also exhibits proof of aberrant development in the large median tubercles of the tergites. Group (c) shows distinct phylogerontic characters in the strong development of the spines, surface scales and other excrescences, as well as in the large epimeral pieces of the postabdominal segments, and these are comprised under the subgeneric term Anthraconectes.

The group Eurypterus sensu stricto, or as represented by division (a), embraces not only the relatively simplest expression of the genus around which the other forms quite naturally group themselves, but also the most vigorous and that which dominates the stage (Salina) where the genus clearly reaches its climacteric development. The Eurypteri of the Bertie waterlime, notably  and , are therefore properly considered as representing the typical expression of the genus.

The genus thus defined is characterized by an elongate, slender body, widest in the middle of the preabdomen and distinctly differentiated into preabdominal and postabdominal regions. The carapace is subquadrate to subrectangular in outline, with rounded anterior angles, relatively small, one fifth to one seventh the total length of the body. The compound eyes are reniform, without distinct facets; situated on the dorsal side of carapace. The ocelli are situated between the compound eyes. The chelicerae are small, not extended beyond the edge of the carapace. The endognathites increase in length from the first backward; the first three pairs relatively robust, short and spinous; the fourth pair slender and bearing only a terminal spine with two spines on the penultimate segment. The fifth pair is developed into swimming legs with bladelike and extended seventh and eighth segments. Its ninth segment is rudimentary. The ventral membrane of the cephalothorax is separated by a distinct suture