Page:The Eurypterida of New York Volume 1.pdf/160

 this Cambric progenitor; the carapace, preabdomen, postabdomen and telson exhibiting the same general characters and number of component parts. Notwithstanding this conformity in structure Strabops presents a number of distinctly primitive characters which indicate its closeness to the archetype of the eurypterids. We see the most important of these in the lack of differentiation in width and length between the preabdominal and postabdominal segments, the contraction of the postabdomen taking place so uniformly that the conventional division between body and tail, so evident in all later eurypterids, is here still entirely undeveloped. The postabdomen, always a slender part of the body with much lengthened posterior segments in later forms, is here short and broad, the segments not at all increasing in length backward. Altogether, all abdominal segments retain a highly primitive and simple aspect in their appearance as uniformly narrow straight bands that hardly bend forward at the lateral extremities. The first tergite, however, has progressed in development beyond the others, and, as in the Siluric genera, is more closely connected with the carapace than with the abdomen, and is distinctly bent forward at the extremities.

The carapace is distinguished from that of the Siluric descendants by its remarkable shortness, or relatively very small size which may also constitute a primitive feature. On the other hand the doublure or rim of the underside is very broad, suggesting the genus Stylonurus. The eyes are especially notable for their small size and circular outline.

The telson spine is short and plump, and thus in correspondence to the short compact body.

Of the limbs but a single fragment is shown, and this is too small to indicate their character. Doubtless they correspond in lack of differentiation with the dorsal test and were probably all of simple and similar form, slightly increasing in length posteriorly. Strabops has served in our effort to reconstruct the archetype of the Eurypterida.