Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/104

Rh slightly constricted at the union of the trunk segments and extends back to the anal segment which protrudes posteriorly from beneath the shield; the gullet connecting the mouth and the stomach must have extended forward and upward.

The large hepatic glands and caeca are somewhat similar to those of Burgessia bella (text fig. 5); a short, strong tube with a wellmarked anterior and posterior tube leads out from the stomach, and branching from these lateral tubes are a series of hepatic caeca. The small hepatic caeca are located between the long posterior tubes of the hepatic glands and the intestine and posterior to the main hepatic tube.

Owing to the excellent state of preservation of many of the specimens showing the hepatic caeca it is probable that they were situated between the dorsal and ventral membrane of the carapace and thus held in position and protected from destruction; that they are preserved at all is one of the wonders of this remarkable Burgess shale fauna.

Functions of appendages.—These were presumably the same as for similar organs in Marrella and Burgessia, and the mode of occurrence is essentially the same.

Diagraminatic restorations.—The diagrammatic restoration (fig. 2) presents the outline of the carapace and posterior shield with the stomach, intestine, hepatic tubes and caeca of the digestive system outlined, also the thoracic trunk, the telson and the thoracic limbs as far as known and interpreted. The data for this diagram are very good except the jointing of the endopodites and the exact form of the coxopodites and proximal joints of the exopodites. In figure 1, the endopodites on each side have been omitted and the exopodites drawn in so as to show their structure and position above the endopodites and below the ventral membrane of the posterior dorsal shield. The sixth limb has both the endopodite and exopodite attached; this should be compared with the thoracic limb of Marrella splendens (text fig. 9).

We know so little of the cephalic limbs of Naraoia compacta that I hesitate to give a diagrammatic sketch of them, and it would not be of even tentative value if we did not have the cephalic limbs of Marrella and Burgessia for suggestion; from the latter and from the evidence afforded by a few specimens, the outline of figure 1 is drawn.

Comparison with crustaceans.—Naraoia has many characters in common with the trilobite and some in common with Marrella, Burgessia, and Waptia, which will be spoken of in the discussion of this group of genera.

Plesiotypes.—U. S. N. M., Nos. 83945a-e.