THE TRILOBITA AND OTHER ARTHROPODA 201 



The labrum is parabolic in outline. From beside it 

 springs a pair of long, unbranched antennae, which are 

 the appendages of the first head-somite, and the only 

 pair not conforming to the general type of the rest. 



FIG. 57. TRIARTHRUS BECKI, GREEN, ORDOVICIAN (UTICA 



SHALE), ROME (NEW YORK). (x2.) 



(After Beecher, modified.) 



The lower figure is a diagrammatic cross-section : on the left side the 

 exopodite with its fringe is alone drawn, on the right the exopodite 

 (without the fringe) and endopodite are shown. (NOTE. The abbre- 

 viations Ex. and End. have been accidentally interchanged.) An., 

 antenna; End., exopodite; Ex., endopodite; F.S., facial suture; 

 Gn., gnathobase ; M. Ap., mouth appendages ; Fl. , pleuron ; Th. Ap., 

 thoracic appendages. 



Any one of the largest appendages (from the first 

 eight thoracic somites) shows the following characters : 

 It is attached to the ventral surface of the body by 

 a joint called the coxopodite, from which a hard jaw- 



