The thorax has seven transverse segments outlined, the anterior of which is somewhat narrower than the others; the segments terminate in blunt, falcate, slightly furrowed ends that overlap on the next posterior segment within the posterior curve of the free portion of the end of the segment. A lobation of the thorax is indicated by the presence of a shallow, elongate depression on seven of the segments one-third of the distance across from the left side; if similar depressions existed on the right side they have been obscured by the flattening of the test in the shale.
The pygidium curves outward from its anterior margin, and the rounded outline extends along the sides and posterior margin; it shows traces of an anterior transverse furrow indicating a segment.
| Dimensions. | mm. |
| Length of dorsal shield | 48.0 |
| Width of cephalon | 17.0 |
| Length of cephalon | 13.5 |
| Greatest width of thorax | 18.0 |
| Length of thorax | 24.0 |
| Width of pygidium | 15.0 |
| Length of pygidium | 12.5 |
Observations.—The only known specimen of this unique fossil was found on a large slab of calcareous shale in association with Bathyuriscus rotundatus (Rominger), Ogygopsis klotzi (Rominger), and Anomalocaris canadensis Whiteaves. The impression of the thin test is very clear and along the left side some of the test remains as a very thin dark scale. The specimen is in the same condition of preservation as the thin tests of the shields and body of Anomalocaris canadensis and was evidently much thinner and more delicate than the tests of the associated trilobites.
Formation and locality.—Middle Cambrian: (14s) Lower portion of the Stephen formation, northwest slope of Mount Stephen, above Field, British Columbia, Canada.
MOLLISONIA GRACILIS, new species
Plate 24, fig. 5
Of this species only one specimen is known. The general outline and proportions of the dorsal shield are shown in figure 5. The specimen has been laterally compressed so that the lateral edges of the cephalon, segments, and pygidium are wrinkled and suggest fine spines.
One of the striking peculiarities is the transverse anterior merging of the cephalon into the short blunt spines projecting from it.
