CRUSTACEA. TRILOBITA 



299 



increase in size backwards : the second pair are considered 

 to represent the antennas, the third the mandibles, and 

 the fourth and fifth pairs the maxillae of other Crustacea. 

 Each maxilla consists of a large basal joint (the coxopodite) 

 which bears a stout endopodite and a slender exopodite; 

 the latter carries a row of hairs or setce ; the inner edge of 

 the coxopodite is toothed and served as a jaw (gnatho- 

 base); whilst the endopodite and exopodite assisted in 

 locomotion. 



Fig. 128. Triarthrus becki, from theUtica Slate (Ordovician) near Rome, 

 New York. (After Beecher.) 



A, view of the ventral surface showing appendages, etc. h, hypo- 

 stome ; m, metastoma. x If. 



B, diagrammatic section through the second thoracic segment. 

 a, endopodite ; b, exopodite. 



C, dorsal view of second thoracic leg. a, endopodite ; b, exopo- 

 dite ; c, coxopodite with gnathobase. Enlarged. 



