Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/115

Rh

Description.—Dorsal shield thin, broadly rounded in front and tapering from the postero-lateral angles of the cephalic carapace to the posterior end of the shield.

Cephalic carapace transverse with the postero-lateral angles extended into spines; posterior margin arched forward; frontal margin reflected to form a doublure to which a small elongate labrum is attached. Eyes unknown but indicated by a bright spot on the carapace a short distance outward from the side of the labrum. No traces of facial sutures.

Posterior dorsal shield, elongate and formed of 14 or 15 fused segments with a more or less distinctly marked border. There is a short transverse segment or telson (pygidium) outlined, but whether it is free from the next anterior segment is unknown.

Surface of test apparently smooth.

Dimensions.—This genus is based on a small animal, S. fragilis, 5 to 17 mm. in length.

Appendages.—There are indications of antennae, three pairs of cephalic limbs, and a pair of limbs for each segment of the posterior dorsal shield.

Digestive organs.—An intestine extends from the posterior segment forward to the central part of the cephalic carapace where it widens out to form an elongate oval stomach. There are traces of hepatic caeca adjoining the stomach.

Genotype.—Skania fragilis Walcott.

Stratigraphic range.—The stratigraphic range is limited to a band of dark siliceous shale about 4 feet in thickness forming a part of the Burgess shale member of the Stephen formation.

Observations.—The generic name is derived from Skana, the name of a glacier in the Mount Robson District, Alberta, Canada.

The specimens representing the species of this genus are small and so thoroughly flattened in the shale that little more than a black film remains. This makes it very difficult to obtain details and also leaves some doubt as to whether the posterior dorsal shield is formed of fused or free segments.

Description.—General outline irregularly heart-shaped but subject to wide variation owing to distortion by compression.