SUBCLASS I 



TRILOBITA 



701 



Pairecl Appendages, — All segments of the cranidium, thorax and pygidium, 

 except the anal segment, carry appendages, all of which are l)iramous 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- 

 podite, which is prolonged 

 into a gnathobase. The ^^^ i343 



inner brancn, or endOpOdlte^ NeoUnus serratuß Romlnger. Middle Canibiiaii ; Burgess Pass, 

 has typically Six jointS. g-C.^ Microphotograph showing elongate x */i 



The outer brauch, 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 



Fig. 1344. 



Triarthriis hrr]:i r;tcen. a, Restored thoracic limbs in transvpisi 

 soction of tlic .uiimal ; }>, Section across anterior portion of pygidium 

 c, Section across ])(jslt'rior portion of pygidium (aftcr Eeoclier). 



Fio. 134Ö. 



Triarthrus becki Green. 

 Dorsal view of second thoracic 

 leg, with and withoxit setae and 

 without gnathobase. e7i, Endo- 

 l)odite ; ex, Exoi)odite (after 

 Beccher). 



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- 



