12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



to the genera Ogygopsis or Orria/ except that the median lobe is not 

 as strongly marked. These two genera are mentioned as they occur in 

 correlative Middle Cambrian formations and one of them in the 

 Stephen formation which is nearly contemporaneous with the Burgess 

 shale. The dorsal carapace and shield also appear similar in outline 

 to the dorsal exoskeleton of the freshly hatched young of Limulus 

 polyphemns as figured by Packard.' The posterior shield has each of 

 the thoracic body segments, excepting the two posterior which pro- 

 ject beyond it, attached directly to it, and it forms the dorsal side 

 of the exoskeleton of the body segments. The exoskeleton of the free 

 posterior segments and telson, of the cephalic segments, and of the 

 ventral side of the trunk segments was so exceedingly delicate as to 

 leave only a slight trace on the shale. Both the anterior carapace and 

 posterior segmented shield were very thin and readily distorted. 



The anterior carapace slightly overlapped the posterior shield and 

 the two were closely held together by the strong body as evidenced 

 by their rarely being found separated. The telson is short and marked 

 by fine short spines. 



Dimensions. — The largest specimen had a length of about 40 mm., 

 the carapace being shortened by compression. A specimen that has 

 been slightly narrowed by compression has the following dimensions : 



mm. 



Length of carapace 16 



Width of carapace 15 



Length of posterior dorsal shield 17 



Width of posterior dorsal shield 14 



Eyes. — The eyes are represented by two small crescent-shaped 

 bright spots a little in advance of the anterior end of the stomach as 

 illustrated by the restoration (fig. 2). The position and form corre- 

 spond quite closely to the paired eyes of the recent Apus lucasanus 

 Packard.^ 



Cephalic appciuiagcs. — The antennae are uniramous, short jointed, 

 and slender in their distal portion, and have a large proximal joint ; 

 the intermediate joints are unknown. Of the cephalic limbs only slight 

 indications were found of the proximal joints of three pairs, and a 

 few terminal joints extending from beneath the carapace, nothing 

 of their original form being preserved. All traces of cephalic ap- 



* Smithsonian Misc. Coll., vol. 64, no. 5, figs, i and 2, pi. 66, 1916. 

 ^ Mem. Boston Soc. Nat. Hist., vol. 2, pi. 5, figs. 25, 2Sd, 1871. 

 ^ Twelfth Ann. Rep., U. S. Geol. and Geog. Surv. Territories, Hayden, Pt. i, 

 pi. xvi, fig. 2, 1883. 



