Page:The Eurypterida of New York Volume 1.pdf/52

 legs, giving the specimen very much the aspect of a spider crab. The first two of these were, however, furnished with a great number of paired spines or leaflike appendages of the underside. It is therefore concluded that in this group, for which the term Ctenopterus is here proposed, a distinct differentiation of the legs has taken place into three spiniferous anterior and two nonspiniferous posterior pairs. Laurie's  is a form in which a like series of at least four pairs of long legs is shown, the two foremost of which bear many long spines [see text fig. 62]. In Stylonurus proper, as represented by, the first three pairs of legs appear to have retained more of their original character, in being relatively shorter and bearing only one pair of spines on each segment. Still another type of differentiation, not represented in our rocks, is shown in  Woodward.

Eusarcus [ pl. 27] represents a distinctly aberrant line of leg development corresponding to the entirely peculiar structure of the animal. The first pair of legs is of the length and character of that of Eurypterus; the second to fourth, however, form a series that decreases in size backward, the second being the longest of the walking legs. Correlated with this marked difference from Eurypterus is the greater length of the anterior spines on each leg. It is manifest that this creature in walking carried the pointed frontal part of its head shield, on which also the lateral eyes are found, raised high above the ground.

In Pterygotus the four pairs of walking legs are simpler than in any other genus. They are of equal length, thin and nonspiniferous and were clearly for walking only, the prehensile function having been entirely transferred to the chelicerae.

It is manifest that in the genera in which the chelicerae are very small, especially in Eurypterus, the walking legs with their long curved spines were, as in Limulus, also actively engaged in grasping prey and transferring it to the chelicerae which transmitted it for mastication to the basal