286 NORTH AMERICAN INDEX FOSSILS. 



middle of the length of the head ; genal spines slightly shorter and 

 broader ; pygidium subsemicircular instead of semielliptical in out- 

 line and with its axis prominently rounded, not pointed. 

 Upper Cambric of New Jersey. 



XXXIV. Triarthrus Green. 



Elliptical. Cephalon semicircular. Glabella large and well de- 

 fined, with straight sides and rounded front, marked by three deep 

 furrows extending toward the center from each side. Eyes small. 

 Central axis of thorax wider than the lateral lobes; furrows of 



Fig. 1589. Dikel- 

 locephalus osceola^ 

 central part of head, 

 X 2, and cheek, X 

 I. (After Hall.) 



Fig. 1590. Triarthrus becki, restoration, showing 

 legs and antennae, upper and under side. (After 

 Beecher. ) 



axis not continuous with those of pleura. Thoracic segments 14 

 to 16. Pleural segments grooved. Pygidium with six segments in 

 the axis and with entire margin. Ordovicic. 



yy. T. fischeri Billings. Ordovicic. 



Differs from T. becki in its small size (length of cephalon 5.5 

 mm.) and in the absence of the tubercles from the axial segments. 



Upper Quebec group, Div. N and P (Chazy), of Newfoundland. 



78. T. becki (Eaton). (Fig. 1590.) Ordovicic. 



Center of each axial segment marked by a tubercle. (Type of 

 genus.) 



