SUBCLASS 1 



TRILOBITA 



701 



Paired Appendages. — All segments of the cranidium, thorax and pygidium, 

 except the anal segment, carry appendages, all of which are biramous save the 

 anterior pair. The anterior 

 antennae, or antennules, are 

 attached at the sides of the 

 hypostoma, and consist of a 

 simple, many-jointed flagel- 

 lum (Fig. " 1342). The 

 caudal rami of the Cambrian 

 genus Neolenus (Fig. 1343) 

 are long, slender, jointed 

 and attached to the last 

 segment of the pygidium. 



The typical Trilobite 

 leg has two branches arising 

 from a basal joint, or coxo- 

 podife, which is prolonged 

 into a gnathobase. The 

 inner branch, or endopodite, 

 has typically six joints. 

 The outer branch, or exo- 

 podite, has a long proximal joint, with a distal multiarticulate portion, or 

 the proximal joint may be flat and elongate, forming the entire exopodite, 

 as in Neolenus (Fig. 1343). Long setae extend posteriorly, and on the distal 

 portion they are so crowded as to make a conspicuous 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 



ti> 



134a. 

 Middle Cambrian 



Neolenus serratus Rominger. 

 B.C. Microphotograph showing elongate setiferous exopodite. 

 (after Walcott). 



Burgess Pass, 



Fig. 1344. 



Triarthrus hccki Green, a, Restored tlioracic limbs in transverse 

 section of the animal ; h, Section across anterior portion of pygidium ; 

 c, Section across posterior portion of pygidium (after Beecher). 



Fig. 134.5. 



Triarthrus hccki Green. 

 Dorsal view of second thoracic 

 leg, with and without setae and 

 without Kuatliobase. en, Endo- 

 podite ; ex, Exopodite (after 

 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 

 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- 



