Page:Walcott Cambrian Geology and Paleontology II.djvu/52

 the cephalo-thorax it has a nearly transverse posterior margin with slightly rounded lateral angles. The anterior outline is curved so as to give an inward slope to the sides and a rounded, slightly transverse section along the central portion. No traces of eyes. I am strongly inclined to the view that the part preserved is the large epistoma characteristic of the family Sidneyidae.

Abdomen.—There are traces of seven abdominal segments. In front of the epistoma? is the remnant of a segment which was largely broken away in exposing the epistoma?. The same is true of the anterior of the segments united in the abdomen; of this segment only a small fragment remains on the left side. The first fairly well preserved segment has a length of 13 mm. and a width of 30 mm. Before the anterior margin was removed it had a length of 16 mm. The next two segments are large and broad, and the last two narrow and long. All are more or less pushed one over the other so as to obscure their true proportions.

Surface.—The surface of all parts of the abdomen is ornamented by irregular, imbricating lines, roughly sub-parallel to the longitudinal axis of the abdomen, or else, toward the outer edges, subparallel to the gently curved outer margins of the segments. The epistoma? has much finer lines sub-parallel to its lateral margins.

Observations.—The outline of the body of this species suggests the form of Pterygotus bilobus Salter var. inornatus Woodward. The surface markings are unlike those of Pterogotus, Eurypterus, and other genera of the Eurypterida, as are also the proportions of the abdominal segments.

Formation and locality.—Middle Cambrian: Stephen formation, Ogygopsis shale, west slope of ridge between Mount Field and Mount Wapta, about 3800 feet above Field on the line of the Canadian Pacific Railway, British Columbia, Canada.

A second species of this genus or a closely allied form occurs in the Redlichia chinensis zone of Indo-China. (See p. 19.) It is the oldest Merostome now known as it comes from the horizon of the Man-t'o shale formation of the upper Lower Cambrian terrane.