28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



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. 



Plesiotypc.—U. S. N. M., No. 83951. 



MARRELLA SPLENDENS Walcott 



Plate 22, figs. 1-9 



Marrella splendens Walcott, 1912, Smithsonian Misc. Coll., vol. 57, no. 6, 

 p. 193, pi. 25, figs. 1-6, pi. 26, figs. 1-6. 



In the preliminary note of 191 2 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 cara- 

 pace 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. 



