616 ARTHROPODA SUB-KINGDOM VII | 
The normal Trilobite leg has two branches arising from a basal joint, or 
coxopodite, which is prolonged into a gnathobase. The inner branch, or endo- 
podite, has normally six joints. The outer branch, or exopodite, has a long 
proximal joint, with a distal multiarticulate portion. Long setae extend 
posteriorly, and on the distal portion they are so crowded as to make a con- 
spicuous fringe, imparting a characteristic appearance to the leg. 
Besides the antennules, the cephalon bears four pairs of pediform biramous 
appendages, with large gnathobases functioning as manducatory organs. Of 

Fic. 1269. 
Triarthrus Becki, Green. 
Fic. 1968 Dorsal view of second thoracic 
ee leg, with and without setae and 
Triarthrus Becki, Green. a, Restored thoracic limbs in transverse without gnathobase. en, Endo- 
section of the animal; }, Section across anterior portion of pygidium ; podite; ex, Exopodite (after 
ce, Section across posterior portion of pygidium (after Beecher). Beecher). 
these the first may be correlated with the posterior antennae of higher Crustacea. 
In structure and function they are true mouth appendages, like the second 
A B 

Fig. 1270. 
Trinucleus concentricus, Baton, Utica Slate (Ordovician); Rome, New York. A, Left half of pygidium 
and three thoracic segments, with test removed, and showing fringes of the exopodites. 2B, Ventral aspect of 
same. «a, Endopodite; b, Exopodite. 1/; (after Beecher). 
pair of nauplius limbs. The second pair, corresponding to the mandibles of 
higher forms, and the third and fourth, corresponding to maxillae, have the 
same structure as the first, with large gnathobases and fringed exopodites. 
The thoracic and abdominal limbs are of the same biramous type. The endo- 
podites are jointed, crawling legs; posteriorly, especially on the pygidium, 
j 
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