76 NEW YORK STATE MUSEUM 
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: 
1 Compound eyes marginal, body slender, fishitke, last pair of limbs 
swimming legs, telson mostly broad and finlike: Hughmilleria, 
Pterygotus, Erettopterus, Slimonia | 
2 Compound eyes marginal and frontal, body scorpioid, last pair of 
limbs swamming legs, telson spintform: Eusarcus 
3 Compound eyes dorsal, subapical, body slender to broad, last pair 
of limbs swimming legs, telson spintform: Eurypterus, Doli- 
chopterus | 
4 Compound eyes dorsal, subapical to apical, body slender, last pair of 
limbs exceedingly long and slender jointed, telson styliform: Drep- 
anopterus, 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 
