Page:The Eurypterida of New York Volume 1.pdf/69

 tergites, as shown by uncompressed specimens [ pl. 20, fig. 8] and by the common observation that in compressed specimens the sternites protrude on both sides from under the tergites [ pl. 4, fig. 4]. Although there are but five sternites including the operculum corresponding to the six tergites of the preabdomen the former are relatively so much longer that they overlap each other with fully half their length.

The sternites, the operculum included, bore the branchiae. Woodward was the first to observe platelike appendages of the sternites in Pterygotus which he described as lamellae, and Laurie observed appendages of the sternites in Slimonia which he designated as "branchial lamellae." Holm found organs in Eurypterus corresponding to those described by Laurie, but considers them only as "Kiemenplatten" instead of "Kiemenblätter," or as oval spongy thickenings of the outside of the thin, soft membrane on the upper side of the sternite, which probably served as attachment places to the branchial lamellae. He has also observed detached bundles of two or three extremely thin superjacent leaves which he considers with doubt as possible branchial lamellae. The "Kiemenplatte" or "branchial plate" exhibits a very characteristic structure consisting of one or two trunk veins running parallel to the longitudinal extension of the plate and from which smaller branches proceed [see text fig. 22].

In our material we have frequently been able to see the impression of the "branchial plates" from the dorsal side, as in the fine specimen of Eusarcus [ pl. 29]. Detached branchial plates have also been observed