Page:Walcott Cambrian Geology and Paleontology I.djvu/36

18 Cambrian and 2,600 feet (792.5 m.) below the Upper Cambrian; northwest slope of Mount Stephen, 3,000 feet (914.4 m.) above the Kicking Horse River, above Field, on the Canadian Pacific Railway, British Columbia, Canada.

Dorsal shield elongate-ovate. Cephalon large, semicircular in outline, about one-fourth the length of the dorsal shield; genal angles extended into spines; cranidium subquadrangular in outline, with long palpebral lobes and narrow fixed cheeks; palpebral lobes elongate, with outer rims continued across the fixed cheeks as narrow ocular ridges; glabella subquadrilateral in outline, with short lateral furrows; strong occipital ring. The facial sutures cut the posterior border within the genal angles and pass inward and slightly forward to the base of the eyes, thence about the palpebral lobe, and forward with slight curvature to the front margin.

Thorax with seven segments; pleuræ terminating in short spines, those of the third or fourth segment in longer spines; pleural furrow with broad inner end largely filled in by an elongated tubercle.

Pygidium large, with central axis divided into several rings, and with the first, or first and second combined, anterior, anchylosed segments extended across the border into a long spine on each side.

.—Albertella helena, new species.

.—Upper beds of Lower Cambrian.

.—Western Alberta, near the line of the Canadian Pacific Railway, Canada, and northern Montana, in the Lewis and Clark Forest Reserve.

.—Albertella is a most interesting type of the order Opisthoparia and family Paradoxidæ. It should first be compared with the genus Zacanthoides Walcott, which, in the British Columbia section, is first met with in strata 2,000 feet above the beds in which Albertella occurs. The cephalons of the two genera are generically the same. The thoracic segments are of the same type, but the third or fourth segment of the thorax of Albertella is extended into long pleural spines, and the thorax has seven instead of nine segments, as in Zacanthoides. The pygidium of Albertella has a long, strong spine extending from the pleural lobes of the first, or first and second combined, anterior segments, and a smooth border otherwise; the