Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/101

Rh Exoskeleton.—The dorsal exoskeleton as seen from its dorsal side is formed of a carapace and a posterior segmented shield. The true cephalic carapace or shell fold is attached to the cephalic somites near its anterior portion, probably as in the recent Apodidae or the associated Burgessia and also along the line of the body as far back as the anterior margin of the posterior dorsal carapace. The carapace is not known to have had a reflected anterior margin with a labrum attached



as in Burgessia, but it may have had, as none of the specimens shows the ventral side of this part in an uninjured condition. The carapace is broader than long, with a somewhat uniformly rounded outline except posteriorly where it is nearly transverse; it probably had a ventral as well as dorsal membrane between which the great hepatic caeca were located very much as are the shell glands in the Apodidae. For some unknown reason the anterior portion of the carapace is usually distorted by being crowded back so as to wrinkle and shorten it.

The large shield of the posterior part of the exoskeleton is composed of 14 fused segments with a narrow border. It has the appearance of the many-segmented pygidium of the trilobite belonging