Abdomen with twelve segments, the anterior nine of which each carry a pair of branchia-bearing appendages. The next two posterior segments, tenth and eleventh, are simple, annular rings, the terminal segment or telson has a central spatulate section that, with its lateral swimmeret on each side, forms a broad caudal fin.
Surface of dorsal shield smooth.
Genotype.—Sidneyia inexpectans, new species.
Siratigraphic range.—The stratigraphic range is limited, so far as known, to a thickness of 130 feet in a dark siliceous shale forming a part of the Stephen formation and described as the Ogygopsis shale in 1908.[1]
Geographic distribution.—On the slope of the ridge between Mount Wapta and Mount Field north of Burgess Pass, and about 3800 feet above Field on the line of the Canadian Pacific Railway, British Columbia, Canada.
Observations.—Sidneyia is a most interesting type and one that I should have expected to find in an Ordovician rather than in a Middle Cambrian formation. It is associated with a large fauna, part of which is enumerated in the list of thirty-two species listed under the description of the Ogygopsis shale referred to above. The stratigraphic horizon in the British Columbia Cambrian section is over 6300 feet below the summit of the Cambrian series.[2]
The short, broad, cephalo-thorax, the broad elliptical abdominal portion formed by the first nine segments, the elongate, narrow three posterior segments, the last taking the form of a broad caudal fin, all unite to give the type a scorpion-like appearance.
The genus dififers from all the genera of the Eurypterida in the form of the cephalo-thorax, smooth surface, presence of a very large epistoma, non-chelate antennae, absence of a metastoma, and (with the exception in Stylonurus) absence of a broad posterior pair of swimming appendages; also in the arrangement of the branchiae upon the nine anterior abdominal segments and in the presence of the broad caudal fin formed of the spatulate terminal section and swimmerets of the twelfth segment.
In this preliminary study and description some detail is omitted, but this will be worked out and inserted in the final study of the genus.
The generic name Sidneyia is proposed in recognition of the discovery of the type specimicns by my son, Sidney S. Walcott, in August, 1910.
