Page:The Eurypterida of New York Volume 1.pdf/71

  ; and moreover is so twisted that while the first caudal segments show the dorsal or ventral side, the last and the telson are seen in profile. This fact, together with the presence of an emargination in the posterior upper margin of the segments and the downward curvature, seems to indicate that the tail was capable of bending upward and forward as in the scorpion.

The section of the postabdomen usually was lenticular at the anterior end, remaining so throughout in some forms, as some species of Eurypterus, while in many species it became circular toward the other end. Circular sections of the caudal segments of species of Eurypterus and Hughmilleria are frequently found [ pl. 63, fig. 12].

The lateral angles of the posterior margins are nearly always produced into pointed lobes. Those of the ultimate segments are usually much larger than those of the preceding segments, as e. g. in. In some species these lobes, together with the lateral edges of the segments, grow out into prominent flat winglike appendages, representing the "epimeral pieces" of the caudal segments. These are notably developed in, in the subgenus Anthraconectes and in some species of Stylonurus.

The ultimate and penultimate segments of Pterygotus carry on the dorsal side a crest or ridge that begins at about the middle and continues to the posterior end. It is doubtless caused by the intestinal canal.

The telson is an appendage of the 12th segment, as indicated by the position of the anus in relation to it. Strabops and our larval stages seem to indicate that the primitive form of this spine was short, thick and four-sided with dorsal, ventral and two lateral edges. This view is supported by the consideration that from this pyramidal form we can most easily derive the two different lines of development of the telson, which culminated on one hand in the styliform telson of Stylonurus and on the other in the bilobed telson of Erettopterus. The more primitive genera, such as Eurypterus and Hughmilleria, retain the general character of the primitive telson and at the same time exhibit in an incipient stage