THE EURYPTERIDA OF NEW YORK 31 
that it entirely covers the legs and is the result of extreme adaptation to 
a mud-groveling habit, but in the eurypterids it never covers more than the 
basal segments of most legs. It nevertheless shows considerable variation 
in relative size, which is roughly correlated to the size and weight of the 
legs. Thus Dolichopterus with its very stout walking legs and extremely 
long swimming legs has a relatively very large carapace, while Pterygotus 
with thin walking and short swimming legs has, in spite of the enormously 
extended chelicerae, a remarkably small carapace. 
The carapace is typically subrectangular in Eurypterus, Dolichopterus, 
Slimonia and Stylonurus, semiovoid to semicircular in Strabops and 
Hughmilleria, and subtriangular in Eusarcus.: The broadly semielliptic or 
semicircular form is manifestly original and primitive, as indicated both 
by the larval stages of the eurypterids and the carapace of the Cambric 
Strabops, while the subrectangular and subtriangular forms are the 
extremes of different lines of. development. 
The carapace culminates at the middle or the posterior third in the 
median ocellar tubercle. Along the margin it is more or less flattened 
and the border is frequently thickened and beveled, forming a shoveling 
edge. This is notably the case in Eurypterus but not in the Pterygotus 
branch of the subclass. 
In the great majority of specimens the surface of the carapace is 
flattened by compression. Nieszkowski figured in Eurypterus a short 
ridge extending backward from the middle of the frontal margin and 
two crescentlike lateral ridges on which the compound eyes are situated. 
Schmidt says that Nieszkowski exaggerated these ridges. He himself 
describes a narrow pointedly triangular prominence reaching from the 
posterior margin forward to the ocellar tubercle, whence two broad sector- 
like elevations extend forward and inclose a median depression extending 
to the frontal margin. Holm has considered the ridges as accidental and 
‘The form of the carapace varies with the position of the eyes, the form of the 
legs and the mode of life of the animals. We shall note these relations more fully 
in another chapter. 
