THE EURYPTERIDA OF NEW YORK 59 
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 atleast four distinct lines of development of the metas- 
toma, which fully correspond to the four principal branches of the euryp- 
terids here distinguished [see chapter on Phylogeny, p. 124] namely, that 
of Eurypterus, of Pterygotus, of Eusarcus and of Stylonurus. 
ale 
VOU 
Figure 17 Metastomas. I, of Eurypterus; II, of Eusarcus; III, of 
‘Dolichopterus: IV, of Sty Jonurus: V, of Hughmilleria ; VI, of Ptery- 
gotus; VII, of Slimonia 
I | II 
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 tazl of earlier writers. The dorsal plates 
of the preabdominal or mesosomatic segments are termed the tergites, the 
ventral pieces the sternites or Blattfusse. Those of the postabdomen are 
known as caudal, postabdominal or metasomatic segments or somutes. 
