Page:The Eurypterida of New York Volume 1.pdf/70

 and in the remarkable specimen of, reproduced in plate 26, figure 2, the branchial plates are distinctly set off from a very tenuous brownish film, the integument of the individual, as thick, jet-black oval plates with a coarsely granular surface. These gill plates were here very much smaller than those of the following sternites, those of the third and fourth sternites being the largest and those of the last pair of sternites again considerably smaller. On the plates of the operculum and of the next sternite, projecting thick-walled tubes can be seen that perhaps correspond to the trunk veins observed by Holm.

Metasoma, postabdomen or tail. The tail of the eurypterids consists of six ringlike segments, which decrease in width and correspondingly increase in length in posterior direction. In the primitive genus Strabops they merely decrease in width; and in all more primitive forms, as Strabops and Hughmilleria, and the simpler species of Eurypterus, the decrease in width is very gradual. In others it is abrupt in the first and second postabdominal segments. The first postabdominal segment in most species is a strongly contracted conical ring. In forms where the preabdomen has been excessively broad, the contraction takes place mainly in the last two tergites and the first postabdominal segment. In Eusarcus, where the contrast between the broad preabdomen and the narrow taillike postabdomen has become extreme, the first caudal segment contracts by more than one half, while the following segments have nearly uniform width, forming a cylindrical tail.

The segments fit into each other like the joints of a telescope, the posterior one always reaching, with its anterior articulating edge, as far as the posterior doublure of the anterior segment. They consequently articulate in all directions and are therefore found either extended straight or curved to either side. An exception to this rule seems to be presented by Eusarcus in which the tail is nearly always curved to one side— although specimens with straight tails have been observed in the