Page:Walcott Cambrian Geology and Paleontology II.djvu/43



The only Merostomes heretofore known from Cambrian rocks are from the Upper Cambrian formations of America. The first discovered was described by James Hall in 1863 as Aglaspis barrandi. Subsequently R. P. Whitfield described a second species as Aglaspis eatoni. This genus was subsequently referred to the sub-order Synziphosura of Packard.

No Eurypterid remains were reported until in 1901 C. A. Beecher described Strabops thacheri from the Upper Cambrian Potosi limestone of Missouri.

Both Aglaspis and Strabops indicated that at the close of Cambrian time the Merostomata had advanced a long way toward a full development of the sub-class and that a series of ancestral forms had preceded them. It has been my desire for many years to discover something of the older Merostome fauna in the Cambrian and thus, if possible, secure further connections between the pre-Cambrian Algonkian crustacean, Beltina, and the great Merostome fauna of the Silurian.

In this paper two genera, Sidneyia and Amiella, are described: the former, from very fine material, and the latter, from one broken and imperfect individual. Both genera appear to belong to a sub-order of the Eurypterida and it may be a distinct order.

When preparing this paper I received from H. Mansuy, Geologist of Indo-China, a series of photographs of Cambrian fossils from Yunnan, and among them one of a fragment of a Merostome showing six segments of the abdomen. From their form and surface markings the species appears to belong to the genus Amiella described in this paper. (See p. 28.)

Classification.—The two new genera, Sidneyia and Amiella, are placed in the new sub-order Limulava of the order Eurypterida, under the new family Sidneyidae. The relations of the order and sub-order are shown in the following tabular view.