THE EURYPTERIDA OF NEW YORK 147 
exhibited in a very remarkable manner by the Siluric Palaeophonus. 
It is quite safe to infer that this is the form of the cephalothoracic 
appendages inherited from the common ancestor. 
A corollary of these inferences is that neither Limulus nor the scor- 
pions is derivable from the eurypterids, but that all three, while related, 
have early separated; and that the eurypterids are still nearest in their 
general aspect to this common ancestor. The early authors in pointing 
) 
‘to the “larval aspect ’’ of the eurypterids, showed therefore, a very clear 
insight into the phylogenetic status of this subclass. The appearance of 
the eurypterids in the Cambric with the essential characters of the group 
is in accordance with this larval aspect while the early separation of the 
scorpions from the stock is evinced by the occurrence of typical scor- 
pions in the Siluric, and by the fact that in the Carbonic they show a 
greater diversity of form than they do today. On the other hand the 
similarity of the Palaeophonus nuncius to recent forms is 
conclusive evidence that the scorpions have been very “ persistent types ”’ 
and have carried their typical characters well back of the Siluric. There 
is no reason to doubt that, as there are eurypterids in the Cambric, the 
scorpions also reach back to that era and the diversion from the common 
ancestor must have already been inaugurated in early Cambric time. 
As to what this common ancestor was we have noclue. The trilobites 
were commonly adduced as competent to furnish it, they, the Xiphosura 
and eurypterids, having been united as “ Poecilopoda,’’ until the phyllo- 
podiform structure of the trilobite limbs was demonstrated by Beecher and 
they were recognized as true, primitive Crustacea. While the trilobites 
are separated bya series of features that effectively characterize them 
as primitive Crustacea (as the protonauplius, the hypostoma, the slender 
jointed antennules, the biramous character of all other limbs, the com- 
pound eyes on free cheek pieces, etc.) and that disprove any assump- 
tion of their ancestral relations to the Merostomata, phylogenists still 
assert that the resemblances between the crustaceans and the Acerata 
(Merostomata and Arachnida) are much closer than those between either 
