Page:Eurypterids of the Devonian Holland Quarry Shale of Ohio.djvu/12

. The last two tergites of the metasoma, however, can be described. The twelfth tergite, or pretelson (fig. 41, a), reveals through the midsection an elevated ridge that becomes more prominent posteriorly.

In the anterior part, this ridge comprises three to four elongated scales which grade into a simple, greatly thickened, elongated scale which occupies the entire ridge at the posterior part of the pretelson. The scales on the lateral parts of the pretelson are densely packed, large, semilunar or concentric in outline on the anterior part of the tergite, grading into more scattered and more sharply pointed scales. On the sides of the central ridge and at the base of the tergite the scales become more elongated. The pretelson is 58.0 mm. long. The preceding tergite, likewise, has a ridge of large scales on its posterior part (see fig. 42). The largest scale is 4.5 mm. long and 5.0 mm. wide. The telson is unknown.

Remarks.—Pterygotus (Pterygotus) carmani differs greatly from other species of the genus. From the New York Silurian Pterygotus (Pterygotus) cobbi Hall, and Pterygotus (Pterygotus) juvenis Clarke and Ruedemann and from the Bohemian Pterygotus (Pterygotus) barrandei (Semper) it differs in the much stouter and less well-developed