36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



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 weak- 

 ness 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 col- 

 lection 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 pi. 22) . The structure of the body and the thoracic appendages is 

 very clearly exhibited, but the cephalic appendages, labrum and cara- 

 pace, are usually so matted together that it is difficult to distinguish 

 the details of structure. 



OBSERVATIONS 



MarrcUa and the frilobitc. — MarrcUa has several characters in com- 

 mon with the trilobite and others that are dissimilar. 



SIMILAR CHARACTERS 



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. 



DISSIMILAR CHARACTERS 



1. Absence uf 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. 



