Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/116

Rh Dorsal shield.—Owing to its extreme tenuity there is usually little more than a dark film on the shale that has definite outlines, and shining through it are traces of the digestive organs and the ventral limbs. The transverse cephalic carapace recalls that of Marrella without the great median spines; it is often incurved at the center of its anterior margin and laterally projects into long backward-curving, spine-like extensions that are so tenuous as to suggest that the cephalic carapace was formed of a delicate membrane.



(✕ 7.) Diagrammatic outline of ventral side showing the various parts as interpreted from several specimens. No details of the segments of the posterior dorsal shield are preserved, but the segments are clearly outlined. The intestine is quite definite, also the fact that it contracted at each segment and expanded into a stomach beneath the cephalic carapace. Only the proximal portion of the limbs is outlined, although fragments of the distal portion are preserved on one specimen.

The thoracic portion of the dorsal shield is clearly segmented in two specimens, and traces of segmentation occur in others, but usually there is only a black smear on the shale with the outline of the intestine showing through it. There appear to be 14 or 15 fused segments and possibly a minute terminal segment or telson. None of the 29 specimens in the collection show the outlines of the median lobe, although one has a slight elevation along the line of the intestine.

Labrum.—Traces of a narrow doublure and small labrum have been seen on two specimens; the labrum appears to have been elongate with an outline similar to that of the labrum of Burgessia.