Page:The Eurypterida of New York Volume 1.pdf/153

 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 scorpions 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 scorpions 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  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 no clue. The trilobites were commonly adduced as competent to furnish it, they, the Xiphosura and eurypterids, having been united as "Poecilopoda," until the phyllopodiform structure of the trilobite limbs was demonstrated by Beecher and they were recognized as true, primitive Crustacea. While the trilobites are separated by a 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 compound eyes on free cheek pieces, etc.) and that disprove any assumption 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