Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/103

Rh are posterior to the hepato-pancreatic tubes passing from the stomach to the hepatic caeca.

There is no clearly defined line between the cephalic and trunk limbs, but from the relations of the limbs in Burgessia and Marrella it is assumed that it is between the third pair of cephalic limbs and the supposed first pair of trunk limbs. The specimens are too much obscured by the compression they have undergone to permit of recognition of the detailed structure of the limbs.

Thoracic limbs.—The specimen represented by figure 3, plate 14, has the distal ends of 17 thoracic or trunk limbs projecting beyond the left margin of the posterior shield; the shield in this specimen has not more than 14 fused segments outlined on it, so it is probable that the three anterior limbs belong with the body segments between the anterior segment of the posterior shield and the third pair of cephalic limbs. Another alternative is that the distal portion of the two anterior limbs extending beyond the margin of the shield belong to the maxilla and maxillula, which would leave only one pair of limbs from the segment anterior to the posterior shield and posterior to the cephalic limbs. The limbs were so subject to displacement, however, that any deduction is very uncertain. The distal portion of the thoracic or trunk limbs shows an endopodite with a slightly curved terminal spine with a slender section back of it corresponding to the slender distal joint of the endopodite of Marrella and Burgessia; and then the joints broaden towards the coxopodite with slight indications of five joints between the distal joint and coxopodite.

The exopodite is represented by many slender filaments that were attached to a multi-jointed arm or support similar in appearance to that of the exopodite of Marrella. The filaments are relatively broad, as they occur flattened on the shale. There are strong indications of large coxopodites, but none show their original form or the exact point of attachment of endopodite or exopodite, and the joints of the endopodites have been so crushed down as to be no longer definitely recognizable. The exopodites were nearly as long as the endopodites, and the filaments of the former are usually extended out to the end of the endopodites or beyond.

Digestive organs.—The exact location of the mouth is unknown, but from the apparent position of the antennae and proximal joints of the cephalic limbs, it was posterior to the point of entrance of the hepatic tubes, back of which the intestine was large with minor hepatic caeca opening into it through four small tubes, all of which are anterior to the posterior dorsal shield as indicated in the diagrammatic restoration (fig. 2); beneath the posterior dorsal shield the intestine is