Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/105

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Three specimens referred to this species are known, of which the one figured shows best the spines on the margin of the posterior dorsal shield; another preserves both the carapace and dorsal shield, the latter having marginal spines while the carapace has a smooth margin. On the third specimen the test of the dorsal shield is nearly all exfoliated on the left side so as to expose the body, several of the fringed exopodites, and coming from beneath them the distal portions of the endopodites.

This species differs from Noraoia compacta in having eight short, small spines on the outer margin of the dorsal shield equally spaced between the anterior margin and a large posterior median spine; all three of the known specimens of the posterior dorsal shield are also larger than those of N. compacta, as they average 25 mm. in length exclusive of the posterior median spine. The one specimen preserving the carapace indicates that it was similar to the carapace of N. compacta, the recognized differences between the two species being confined to the posterior dorsal shield.

Holotype.—U. S. N. M., No. 83946.


 * Burgessia bella Walcott, 1912, Smithsonian Misc. Coll., vol. 57. no. 6, p. 177, pl. 27, figs. 1-3; pl. 30, figs. 3-4.

Since the publication of the original description of Burgessia bella Walcott, a large number of more or less well preserved specimens have been collected from the Burgess shale, a few of which preserve details of structure that make it possible to draw a diagrammatic restoration indicating the increase in our information of the cephalic and thoracic appendages (text figs. 3, 4).

Exoskeleton.—The exoskeleton is very delicate and the carapace is so thin as to be almost membranaceous. The segment to which the telson is attached appears to be partly covered ventrally by a heart-shaped plate that is attached to the anterior margin of the segment or to the posterior limb-bearing segment of the thorax; it suggests a supra-anal plate.

There appear to be five cephalic, eight thoracic, and one abdominal segment, also a long, slender telson with numerous joints. One example 21 mm. in length has 30 joints. 2