ORDER II OPISTHOPARIA 629 
Ellipsocephalus, Zenker (Fig. 1298). Cephalic shield semicircular; glabella 
smooth, obtusely angular in front; free cheeks short and narrow; thoracic segments 
twelve to fourteen; pygidium short, rudimentary. Cambrian; Europe and North 
America. 
Sao, Barr. (Figs. 1278, 1299). Glabella slightly tapering forwards, extending 
two-thirds the length of the cranidium, and having three pairs of distinct lateral 
furrows ; thoracic segments seven- 
teen; pygidium very small. 
Cambrian ; Europe. 
Triarthrus, Green (Figs. 1274, 
1300). Glabella large and well- 

Fic. 1298. 
Ellipsocephalus Hoff, 
Schloth. Cambrian ; 
Ginetz, Bohemia. 

= Fic. 1300. Fic. 1301. 
Bosh ED Triarthrus Becki, Green. With _ Dikelocephalus Minnesotensis, D. 
Sao hirsuta, Barr. antennae and legs. Ordovician ; Owen. Cambrian; Wisconsin. 
Cambrian; Skrey, Rome, New York... 9/g (after A, Cranidium. 8B, Pygidium (after 
Bohemia. Beecher). Hall). 
defined, furrows not continuous; thoracic segments fourteen to sixteen; pleura 
grooved; pygidium with six segments in the axis, margin entire. Ordovician ; 
America and Scandinavia. 
Dikelocephalus, Owen (Fig. 1301). Cephalon large, crescentic; glabella not ex- 
tending to frontal margin ; eyes well developed ; thoracic segments nine. Pygidium 
very large, broad and fan-like, with the sides produced posteriorly into spines, axis 
with four to six rings, extending less than two-thirds the length of the pygidium. 
Cambrian ; North America and Europe. 
Family 3. Asaphidae. Emumrich. 
Cephalon and pygidium well developed ; glabella often obscurely limited. Free cheeks 
usually separate. Facial sutures extending forwards from the posterior edge of the 
cephalon within the genal angles, and cutting the lateral or anterior margins, occasionally 
uniting in front of the glabella. Eyes usually present, smooth, well developed, sometimes 
of very considerable size, even occupying the entire surface of the free cheeks. Thorax 
generally composed of eight or ten segments, but varying from five to ten. Pygidiwm 
large, often with wide doublure. Cambrian to Silurian. 
Asaphus, Brongt. (Cryptonymus, Eichw.), (Figs. 1265, 1288, C, 1302). Cephalic and 
caudal shields of nearly equal size, with broad infolded margin; glabella expanded, 
