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 for the articulation with the next segment. The gnathobase is a strong trapezoidal lobe connected by a contracted neck with the main body of the coxa. On the inner or slightly curved edge, it bears 13 relatively blunt teeth which decrease in size posteriorly with a slight backward direction. The first tooth is distinctly the largest and diverges from the others, and the last teeth at the posterior end (the 14th and 15th of the other species) coalesce into a rounded lobe. The circular opening near the posterior

border of the coxae which occurs in Limulus, Eurypterus and Hughmilleria, has not been seen in Pterygotus.

The second segment is short and ringlike, broadest on the posterior side and provided with a concave outer margin. The third appears wedgelike and is broadest on the anterior side; the fourth is again ringlike, broader than the preceding segment and broadest on the posterior side. The fifth segment is quite like the corresponding part in, triangular semioval in shape, the anterior margin the longest and the distal margin deeply notched; the sixth segment is irregularly rhomboidal