Page:The Eurypterida of New York Volume 1.pdf/72

 the new features that lead to Stylonurus and Erettopterus. In Eurypterus also the telson has still the original four-sided form, but the two upper sides have already become so reduced that they are united in either a flat or a concave broad dorsal side, while the ventral edge is developed into a flat-topped carina. In Hughmilleria the development is reversed, the dorsal side bearing a carina, and the ventral side being smooth and either flat or but slightly convex, the telson having thus the subtriangular section and dorsal carina of that in Limulus. The essential characters of the telson of Eurypterus are retained in Dolichopterus. In Anthraconectes, however, it becomes extremely long and styliform, thus assuming the characters in Drepanopterus and Stylonurus. In the extreme forms of the latter genera the telson becomes contracted in the proximal portion and expanded clublike in the distal portion, sometimes with development of flat lateral carinae or flanges, as in.

The telson of Eusarcus was apparently still four-keeled; but it was bent downward toward the end, so that when the scorpionlike tail was thrown forward over the body, its sharp point would be directed upward. This is an aberrant development of the telson not found in other genera, but clearly connected with that of Eurypterus.

From the telson of Hughmilleria first that of Slimonia, and through the latter, those of Pterygotus and Erettopterus can be derived. Slimonia possesses a long tail spine with dorsal carination. The anterior portions of the lateral carinae of this spine, however, are developed into two broad flanges which together form an oval, leaf like expansion with coarsely serrate posterior margin, the median keel being continuous with the long spine [see text fig. 25]. In Pterygotus the projecting spine of the Slimonia stage has been reduced to the size of the serrations of the flanges, and in Erettopterus the reduction of the median spine that has become the axis of the broad leaflike telson has been carried still further so that a bilobed telson has resulted.

The keeled lateral and ventral edges of the tail spine in Eurypterus and the lateral edges of the broad telson in Pterygotus and Erettopterus