Page:The Eurypterida of New York Volume 1.pdf/61

 the metastoma has become subtriangular or cordate in outline, the anterior portion being greatly widened and emarginate and the posterior tapering to a blunt point.

We have thus at least four distinct lines of development of the metastoma, which fully correspond to the four principal branches of the eurypterids here distinguished [see chapter on Phylogeny, p. 124] namely, that of Eurypterus, of Pterygotus, of Eusarcus and of Stylonurus.

Abdomen. The abdomen consists of 12 segments or somites. The anterior six of these are divided into separate dorsal and ventral pieces. They form the preabdomen or mesosoma which is also sometimes termed the thorax; the six posterior ones are annular and form the postabdomen or metasoma, the abdomen or tail of earlier writers. The dorsal plates of the preabdominal or mesosomatic segments are termed the tergites, the ventral pieces the sternites or Blattfüsse. Those of the postabdomen are known as caudal, postabdominal or metasomatic segments or somites.