SUB-CLASS I MEROSTOMATA 675 
is the cleft-shaped mouth, which is bordered laterally by the basal segments of the 
fifth pair of legs, and posteriorly by the large oval metastoma. 
The first pair of appendages were regarded by Woodward and Schmidt as filiform and 
tactile. Laurie and Holm, however, have found evidence that they are chelicerate, and thus 
more nearly in accord with homologous structures in other members of the family. The three 
succeeding appendages are six- or seven-jointed, and covered with fine spines. The fifth pair 
of legs is eight-jointed, and longer than those in front of it. The posterior pair is a powerful 
swimming organ; its great sub- 
quadrate basal joints enclose the 
metastoma, and together with this, 
cover nearly one-half of the ventral 
side of the cephalothorax. 
The six anterior abdominal seg- 
ments together occupy about one- 
fourth of the body-length. They 
are short and broad, and _ nearly 
uniform in shape; but the second 
segment has lost its hard skeleton 
on the ventral surface, and the first 
covers the greater part of the genital 
operculum. This first segment joins the posterior 
margin of the cephalothorax and consists of two lateral 


Fic. 1418. 
Eurypterus Fischeri, Eichw. Silurian; Rootzikiill, Island of 
Oesel. Ventral aspect. II.—VI., Appendages with the joints Fic. 1419 
numbered. I’.—V’., Lamellar appendages of the under side of Aig a 
abdomen, divided by median suture. I’., First of these, or Eurypterus Fischeri, Eichw. Silurian ; 
“operculum,” composed of two portions (b, b’). For abdominal Island of Oesel. Female individual, 1/2 
segments after 7 see next figure. 1/; (after,F, Schmidt). (from Holm). 
portions and a median process. The first six segments are moderately overlapping on the 
ventral side, and divided by a median suture or cleft into two parts. The following six 
abdominal segments are ring-like, tapering posteriorly, and the body is terminated by a long 
slender telson. 
About twenty-five species of Eurypterus are known, the largest being about 40 cm. long. 
They are found for the most part in argillaceous or sandy deposits in transition strata between 
the Silurian and Devonian of England, Gottland, Oesel, Podolia, and in the Waterlime Group 
of New York. They are rare in the Devonian, but occur again more frequently in the Coal 
Measures of Scotland, Silesia, Bohemia, Saarbriicken, and Pennsylvania. The last survivor 
