Page:The Eurypterida of New York Volume 1.pdf/33

 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