THE EURYPTERIDA
I
MORPHOLOGY, ANATOMY AND TERMINOLOGY
General form. The body of the eurypterids, as a rule, is elongated, and often somewhat fishlike in dorsal view, but it may also become distinctly scorpioid. The slender fishlike form is typically expressed by Hughmilleria [see restoration, pl. 59] where the body expands but slightly in the anterior third and then tapers very gradually to the caudal spine; the head shield is semiovoid and the body less depressed than in other genera. Pterygotus is similarly built, but the head shield is more rounded in front and some species are broad and plump. In Eurypterus the lateral expansion or flattening of the body becomes manifest in both head shield and abdomen and the contraction of the body to the caudal portion is more abrupt. Stylonurus has a slender body which, however, expands gradually beyond the middle and then contracts more rapidly. In Slimonia a scorpioid appearance is produced by the squarish head shield and the long tubular caudal portion of the abdomen. An extreme differentiation from the slender terete body of Hughmilleria is reached in Eusarcus with its triangular head shield, broad flat body with subcircular outline, from which the long narrow tail is sharply set off.
We shall recur to this variant expression of the body and its bearing on the mode of life of these animals after a consideration of the appendages which are correlated to the form of the body and corroborate the evidence from the latter.
Integument. The body is covered by a chitinous exoskeleton which alone is preserved in the rocks and usually compressed into an extremely tenuous, carbonaceous, more or less wrinkled film. Notwithstanding its thinness it must have been, like the tough leathery integument of Limulus, very strong and able to furnish a stout basis for the powerful muscles of the creatures.
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