On some Points in the Structure of the Xiphosura , having reference to their relationship with the Eurypteridæ

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Abstract
So long since as 1849* Professor M'Coy made the proposition to unite in one tribe of the order Entomostraca the recent and fossil Limulidæ and the extinct species of Pterygotus and Eurypterus . “The tribe Pœcilopoda,” he observes, “are to be distinguished from other Entomostraca by having crustaceous, didactyle, ambulatory thoracic feet, as well as membranous, respiratory abdominal ones.” Professor M'Coy divides the Pœcilopoda into two divisions:—1. Limulidæ— Limulus ; and 2. Eurypteridæ— Eurypterus, Pterygotus , and Belinurus . He subsequently furnished a restoration of Pterygotus problematicus , in illustration of his view of the anatomy of this ancient fossil remain†. It is not surprising that Professor M'Coy should have failed to establish this order, since it was shown by Professor Huxley‡ to be founded upon an erroneous interpretation of the fossil remains; nor can it be doubted that the arrangement was based on conjecture rather than upon any minute acquaintance with the anatomy of these extinct forms of Pterygotus and Eurypterus , then only known in England by extremely fragmentary remains. The researches, too, of Professor Huxley§ into the anatomy and affinities of the genus Pterygotus (in 1859) do not favour M'Coy's view of their classification under a common order or tribe with Limulus , although he does admit that they possess several very important points of structure in common with the latter. “The Pœcilopoda” (i. e. Limulidæ), observes Professor Huxley§, “are, I believe, the only Crustacea which possess antennary organs like those of Pterygotus , and, like them, have the gnathites converted into locomotive organs, want the appendages

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