On comparison of the characters similar to the eurypterids, the king crab and the brachyuran crabs here mentioned, we may first remark that both. Limulus and the crabs are highly specialized in comparison with the eurypterids. This is evinced in Limulus by the extreme broadening of the carapace, the adaptation of the last pair of legs to burrowing and the fusion of the abdominal somites; in the crabs by the excessive broadening of the cephalothorax and the reduction of the abdomen. In both cases the specialization is mainly a distinct adaptation to the crawling and burrowing habit. No such far-reaching specialization is found among the eurypterids.
In surveying the genera of the eurypterids in regard to the characters bearing on their habits, we find that they readily fall into four groups which show the following differentials:
- Compound eyes marginal, body slender, fishlike, last pair of limbs swimming legs, telson mostly broad and finlike: Hughmilleria, Pterygotus, Erettopterus, Slimonia
- Compound eyes marginal and frontal, body scorpioid, last pair of limbs swimming legs, telson spiniform: Eusarcus
- Compound eyes dorsal, subapical, body slender to broad, last pair of limbs swimming legs, telson spiniform: Eurypterus, Dolichopterus
- Compound eyes dorsal, subapical to apical, body slender, last pair of limbs exceedingly long and slender jointed, telson styliform: Drepanopterus, Stylonurus
We are disposed to believe that these four groups of genera represent four different modes of life habit.
In regard to Pterygotus and Erettopterus, the typical representatives of the first group, it has been remarked by Woodward that the marginal eyes which are half on the underside, prove that the animals could not have been mud grubbers and that the spatulate and bilobed telson was obviously adapted to swimming. Laurie [1893, p. 521] has also conceded that "the only advantage which occurs to one as possibly appertaining to
