Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/118

Rh associated, in the character of the thoracic segments and pygidium; also, so far as we can determine from this superficial study, there are seven segments and the pygidium shows distinct segmentation with a denticulated border."

A complete individual was found after the two fragments were described in 1912. The angularity of the shield at the bottom of the specimen as mounted on the plate is characteristic as its essential angles and curves are repeated in all the specimens referred to the species.

Plesiotype.—U. S. N. M., No. 83951.


 * Marrella splendens Walcott, 1912, Smithsonian Misc. Coll., vol. 57, no. 6, p. 193, pl. 25, figs. 1-6, pl. 26, figs. 1-6.

In the preliminary note of 1912 the general form and appearance of the carapace and appendages of Marrella splendens were described and illustrated. Since then a large number of specimens have been collected, some of which have added to our information both of the carapace and ventral side.

Exoskeleton.—The exoskeleton with the exception of the carapace is very delicate and formed of a series of 31 segments or somites, to 24 of which a pair of biramous appendages are attached; also a terminal segment of the body forming a minute plate-like telson and five segments of the head indicated by the presence of four pairs of free appendages and one segment incorporated in the body of the carapace; this is indicated by the anterior lateral free spines of the carapace with a pair of sessile eyes. As far as may be determined from the compressed fossil specimens the section of the body segments was broadly oval with a dorsal stergite and a ventral sternite section, the appendages being attached on the lower side on the ventral sternite below the margins of the dorsal stergite.

This does not mean that the eyes necessarily represent the anterior segment but that they represent one segment whatever may have been its original position.

Carapace.—Carapace strong, subquadrangular, and with two large dorsal postero-lateral, spinelike lobes (fig. 9) comparable with the postero-lateral lobes of the carapace of the Apodidae. At each antero-lateral angle a strong, backward-curving spine is attached by a close suture. These spines complement the great dorsal thoracic spines and may be compared with the movable or free cheeks of the trilobite. A narrow median carina or ridge extends the entire length of the lateral spines and the postero-lateral lobes.