Page:The Eurypterida of New York Volume 1.pdf/62

 The preabdomen is widest at the fourth or fifth tergite whence it usually contracts more rapidly. It formed a unit in the movements of the body, the easier articulation taking place between it and the cephalothorax on one side and at the boundary of preabdomen and postabdomen on the other. The first tergite is a narrow plate curved backward and with rounded ends. The other tergites are transverse bandlike plates, with convex anterior and concave posterior margins in the middle and the lateral ends curved slightly forward. Corresponding to this outline of the plates the middle portion of the preabdomen is elevated forming the rhachis, while the wings are often depressed or concave. This lateral portion of the segment is frequently termed the epimeral portion, epimera or pleura [ pl. 5, fig. 3]. The epimera are produced at the antelateral angle into lobes, or ears, especially distinct in Pterygotus. These "ears" have been considered as serving for the attachment of muscles but according to Schmidt they only correspond to the rounding of the postlateral angles and served merely to protect the outside of the body. The lateral and posterior margins are furnished with more or less broad doublures to which the connecting membrane is attached. The anterior margin of the tergites which is overlapped by the preceding one (except that of the first tergite) is smooth or bears only very fine ornamentation and is depressed, thus forming an articulation with the doublure of the preceding segment. In some genera, as Eurypterus, this articulation extends the whole width of the tergite; in others, as Pterygotus, only the rhachis is provided with a distinct articulation; this difference probably indicating different degrees of mobility of the preabdomen.

The posterior margin of the gliding or articulating face of the tergite is mostly bounded by a continuous transverse line of scales [ pl. 8, fig. 2].

Corresponding to the six tergites are only five ventral plates or sternites. This is due to the fact that the first two ventral segments lack the ventral sclerites, their place being taken by the large genital plate or operculum, homologous to the operculum of Limulus which bears the generative organs. The operculum of the eurypterids consists of a pair of plates meeting in