146 NEW YORK STATE MUSEUM 
some of which lie in the form of the carapace, the early embryonic differ- 
entiation of the pedipalps from the other appendages, the strong prelarval 
differentiation of preabdomen and postabdomen and the disappearance 
of the abdominal appendages in the scorpions and their persistence in 
the eurypterids. 
It is yet to be determined whether these differences are of phylo-. 
genetic significance or only due to the pushing back, by acceleration, of 
later adaptations of the scorpions into the embryonic and larval stages. 
The latter is undoubtedly the case in the embryonic development of the 
pedipalps and of the narrow, distinctly defined, scorpionic tail. The early 
appearance and later atrophy of the abdominal appendages is, however, 
clearly a feature that points to a common ancestor for the scorpion and 
the eurypterids having such appendages, and we believe that the cepha- 
lothorax in the embryo of the scorpion, retains ancestral features from 
the facts that its length corresponds to about six abdominal segments 
and it equals the latter in width [text fig. 29]; that, however, the strong 
development of the procephalic region is tachygenetic. 
A comparison of the larvae of all three, the: eurypterids, Limulus 
and the scorpion, shows both the latter to have lost the primitive form 
of the abdomen by acceleration, that of Limulus being much broadened, 
that of the scorpion abruptly contracted to the tail or postabdomen while 
the eurypterids have best preserved the original gradual and uniform 
contraction. The carapaces of the eurypterids and the scorpion have most 
nearly retained the original proportions and form of the common ancestor. 
Of the cephalothoracic appendages the chelicerae are alike in all three 
groups and obviously ancestral in their form; the remaining legs have 
taken quite different courses of adaptation, the scorpions having developed 
the powerful chelate pedipalps, the eurypterids the swimming legs, while 
those of Limulus have remained relatively undifferentiated, and show no 
tachygenetic features in the embryos except the chelae. The embryo of 
the scorpion shows simple walking legs, like those of the eurypterids, and 
“lacks the two movable claws. This simple form of the walking leg is also 
