Page:Walcott Cambrian Geology and Paleontology II.djvu/278



General outline elongate, sides subparallel; cephalon and pygidium subequal in size and outline, and indistinctly lobate as now known. Eyes unknown. Thorax with seven simple transverse segments that may or may not be divided into axial and lateral longitudinal lobes by slight depressions on the line of each third of the segment. Surface finely granular.

Genotype.—Mollisonia symmetrica, new species.

Stratigraphic range.—Middle Cambrian: Burgess shale member of the Stephen formation, British Columbia.

The generic name is derived from Mollison, the name of a mountain southwest of Field on the Canadian Pacific Railway, British Columbia, Canada.

Observations.—In its subequal cephalon and pygidium this genus resembles Agnostus and Microdiscus, but it differs in the absence of distinct lobes and the greater number of thoracic segments. The almost unsegmented cephalon and pygidium and few simple thoracic segments are characters of the most highly developed families of the Trilobites, such as Asaphidæ and Illaenidæ, but in the absence of satisfactory evidence of the presence of eyes on the cephalon further study of better material is needed. The form of the thorax recalls that of Bohemilla stupenda Barrande.

General outline elongate, length about three times the width; sides subparallel; cephalon and pygidium nearly equal in size and contour; thorax with seven segments. Test thin with a minutely granular surface.

Cephalon a little shorter than its width at the posterior margin; sides nearly straight and sloping slightly inward toward the broadly rounded antero-lateral angles and front margin. The presence of eyes is very doubtfully suggested by small, faint, crescentiform depressions about 2 mm. from the antero-lateral margin. A raised line that possibly may be the facial suture extends back to the margin midway of the length of the cephalon; in front of the possible eye lobe it appears to pass across the front parallel to the margin and about 1.5 mm. from it. There are faint traces of transverse furrows indicating the presence of about five transverse lobes on the central portion of the cephalon.