THE TRILOBITA AND OTHER ARTHROPODA 203 



there is evidence of such gills. The only indication of 

 specialization in Triarthrus is that in the four pairs of 

 head-appendages (behind the antennae) the gnathobases 

 are larger, and the exopodite and endopodite smaller, so 

 that (as might be expected in the neighbourhood of the 

 mouth) the jaw-function is beginning to predominate 

 over the other functions. 



4. Trinucleus concentricus (Fig. 58) is a trilobite 

 found in the Upper Ordovician Shales of Shropshire 

 and Wales. Complete specimens are rarely found, but 

 the usual (though not maximum) length appears to be 



FIG. 58. TRINUCLEUS CONCENTRICUS EATON, CARADOCIAN. 

 (Natural size.) (After Phillips.) 



about 17 mm., of which 9 -5, 4-5 and 3 mm., measure head, 

 thorax and pygidium respectively. The greatest width 

 of the thorax is 12 mm., that of the head 18 mm. This 

 great difference is due to the presence of a remarkable 

 flat brim projecting from the free margin of the head, and 

 perforated by a number of pits, arranged in four or five 

 roughly concentric rows. This brim is about 3 mm. 

 wide at each genal angle, where it contracts into a long, 

 outwardly-curved genal spine, which extends back 4 or 

 5 mm. beyond the end of the pygidium ; at the front 'end 

 of the head the brim is about 2 mm. wide, so that the 

 length of the head without the brim is about equal to that 



