58 NEW YORK STATE MUSEUM 
would at once have seriously interfered with the movements of the crea- 
tures in several ways; one, because the line of fusion would be directly 
under the important articulation between the carapace and preabdomen, 
and another, it could not have failed to disturb the mutual independence 
of two organs of entirely distinct character and rhythm of movement, 
namely, the coxae of the swimming legs which the metastoma closely 
adjoins, and the operculum with its respiratory and sexual functions. 
The form of the metastoma has been found by us to be highly char- 
acteristic of the genera of the eurypterids and to be a good indicator of 
their phylogenetic relations. That of Eurypterus is typically oval in 
Figure 15 Eurypterus 
fischeri  FEjichwald. 
Metastoma, showing the 
deep furrow of the an- Figure 16 Diagram of sagittal median section through Eurypterus: 
terior portion. (From 7, metastoma; 8, operculum; 2, 3, 4, 5, endognathites. (Gaskell’s 
Holm) reconstruction) 
outline, while those of Hughmilleria and Pterygotus show a strong tendency 
to become wider in the anterior half and narrower in the posterior, and 
at the same time more deeply emarginate in front. In Shmonia this 
tendency is carried to an extreme, the metastoma having become very 
elongate-cordate with a narrow posterior half and a deeply emarginate 
anterior one. In Dolichopterus and the Stylonurus branch in general, 
again a different tendency is developed. Here the base of the metas- 
toma becomes rectangular, the lateral margins subparallel and the plate 
approaches a long rectangle, with the short front side deeply emarginate 
(Stylonurus) or it becomes lyrate (Dolichopterus). Finally, in Eusarcus 
