Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/102

to the genera Ogygopsis or Orria,^[1] 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 polyphemus as figured by Packard.^[2] The posterior shield has each of the thoracic body segments, excepting the two posterior which project 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 correspond quite closely to the paired eyes of the recent Apus lucasanus Packard.^[3]

Cephalic appendages.—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-

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1. Smithsonian Misc. Coll., vol. 64, no. 5, figs, 1 and 2, pl. 66, 1916.
2. Mem. Boston Soc. Nat. Hist., vol. 2, pl. 5, figs. 25, 25d, 1871.
3. Twelfth Ann. Rep., U. S. Geol. and Geog. Surv. Territories, Hayden, Pt. 1, pl. xvi, fig. 2, 1883.