Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/117

Rh Dimensions.—The largest specimen has a length of 17 mm., but the average length is from 5 to 8 mm.

Appendages.—One specimen has several thoracic endopodites out of place on one side, and other specimens show the proximal joint obscurely but sufficiently well to recognize them; another specimen has what may have been slender antennae projecting from beneath the flattened labrum and posterior to it three pairs of slender appendages in which all traces of joints have disappeared; there are also on this specimen several threadlike, silvery lines extending from the central axis out to a margin which indicates that the limbs were long and slender; none of the specimens clearly show the exopodites or any details of the limbs. Specimens of Marrella and Burgessia often have threadlike, silvery lines representing the limbs, these lines being the pyritized contents of the joints, the test having disappeared in the process of mineralization of the original specimen.

Digestive organs.—The stomach is represented by an enlargement of the anterior portion of the intestine within the cephalic carapace, and the intestine extends back to the last segment; traces of hepatic caeca also occur beneath the cephalic carapace adjacent to the stomach.

Observations.—This very delicate form was placed, when sorting the collections, among specimens of the young of Marrella splendens, but it became evident upon close examination that they were quite distinct. They have a dorsal shield resembling that of Naraoia. I have examined all the specimens in hopes of finding free segments, but without results. There is no well-defined border about the posterior dorsal shield as in Naraoia, but there is a definite margin that is unbroken by the extension of the fused segments beyond it.

The almost complete flattening of all the specimens prevents any comparison with the median lobe of the trilobites, and there is no indication of facial sutures although there are slight traces of eyes on the cephalic shield at about the same place as in Nathorstia.

Holotype.—U. S. N. M., No. 83950.


 * Mollisonia ? rara Walcott, 1912, Smithsonian Misc. Coll., vol. 57, no. 6, p. 198, pl. 24, figs. 6, 7.

Original description.—"Of this species there are several fragmentary specimens. The species dififers from M. gracilis, with which it is