Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/126

Rh thin film and matted those parts resting on each other together unless there was a thin film of mud between them. When there was such a film of mud, it later hardened into shale and formed a plane of weakness along which the shale parted when, from the action of weathering or of force applied with hammer or chisel, the shale was split open. Sometimes the parting is between the matrix and the ventral or dorsal surface of the specimen, or it may be between the series of fringed exopodites and the endopodites; a number of specimens in the collection show a part of the endopodites with the exopodites above or below them, and again the parting may have been above or below the exopodites on one side of the body and the reverse on the other side (see pl. 22). The structure of the body and the thoracic appendages is very clearly exhibited, but the cephalic appendages, labrum and carapace, are usually so matted together that it is difficult to distinguish the details of structure.

Marrella and the trilobite.—Marrella has several characters in common with the trilobite and others that are dissimilar.


 * 1) A cephalic shield supporting a labrum.
 * 2) Sessile eyes on the proximal end of a great spine equivalent to the free cheek of the trilobite.
 * 3) A labrum (hypostoma) with the proximal joints of the cephalic limbs gathered at its posterior end in a manner comparable with that of the trilobite.
 * 4) A pair of biramous limbs for each trunk segment formed of a protopodite, jointed endopodite (leg), and a jointed exopodite, but without any known epipodite.
 * 5) Expansion of the joints of the endopodites on some of the thoracic limbs.


 * 1) Absence of a thoracic dorsal shield.
 * 2) Almost total absence of an abdominal section or pygidium.
 * 3) Position of proximal joint of antennae.
 * 4) A large third cephalic appendage (mandible).
 * 5) The manner of attachment of the coxopodite of each trunk limb directly by its proximal end to the side of the ventral surface of the body.