Carapace.—Carapace semicircular, with a deep notch on the posterior side. It appears to have had an upper (dorsal) and lower (ventral) membrane between which the irregular ramifications of the hepatic caeca were located. The shell glands, so conspicuous in the Apodidae, have not been recognized in Burgessia.
Labrum.—The labrum is attached to the reflected anterior rim (doublure) of the ventral side of the carapace and extends back nearly one-third its length; the labrum is rounded posteriorly and has a shallow obliquely transverse furrow on each side just in advance of its posterior margin; it was thin, readily distorted by pressure and is rarely preserved; one of the best examples is illustrated by figure 3, plate 17; it appears to have covered the anterior portion of the mouth.
Eyes.—The eyes are indicated by a minute round spot on each side of the dorsal median axis of the carapace and a short distance within the anterior margin.
Fig. 4.—Diagrammatic outline of a thoracic limb of Burgessia.
pr, protopodite; en, endopodite; ex, exopodite.
The outline is based on the examination of many specimens aided by the known form of the endopodite of Marrella which is somewhat similar.
Dimensions.—The average length of the larger specimens is about 10 mm. A few are 12 and many are 6 to 8 mm. in length exclusive of the long telson. The relative proportions of the carapace, thorax, and abdomen are indicated by the diagrammatic restoration (text fig. 3).
Cephalic appendages.—These consist of well-marked antennae (figs. 3, 4) and three pairs of limbs situated between the antennae and the hepato-pancreas tubes; there is evidence that the basal or proximal joints of the cephalic limbs are relatively large and the remaining joints slender, but their exact position in relation to the labrum and their details of form and structure are not determined. It is quite probable that they represent the mandibles, maxillulae, and maxillae very much as in Marrella, and I have so represented them in the diagrammatic restoration of the ventral view of the species (text fig. 3).
Thoracic limbs.—The ten pairs of biramous thoracic limbs are uniform in character with the exception of the posterior pair, which are
