313 



thorax, width equal to length; surface scarcely trilobate 

 and axis very broad ; pleurse narrow, deflected along their 

 median line; sub-triangular,. faintly trilobate pygidium, the 

 annulations of which become obsolescent at maturity ; sur- 

 face pitted. 



Found at " Hamburg and Eighteen Mile Creek, Erie 

 County." (Hall and Clarke.) It occurs in the lowest Trilo- 

 bite bed, and in the shale below it, down to and in the 

 Pleurodictyum beds of Avery's Ravine. It is a rare form, 

 and no perfect specimens have so far been obtained from 

 this region. 



Genus PHACOPS. Emmrich. 



[Ety. : Pliakos, lens; ops, eye.] 

 (1839: Emmrich.— De Trilob. Dissert.) 



"Body oval. Cephalon parabolic; genal angles obtuse, or 

 produced into minute spines. Glabella tumid, prominent, 

 widest anteriorly ; the two anterior pairs of lateral furrows 

 indistinct. Eyes very large, con- 

 spicuous, bearing numerous cor- 

 neal lenses. Thorax sub-quad- 

 rate; segments eleven ; pleurae 

 arched, and rounded at their ex- 

 tremities. Pygidium moderately 

 large, composed of few annula- 

 tions ; margin entire and not pro- 

 longed into a terminal spine." 

 (Pal. N. Y., Vol. VII., p. xxix.) 



Phacops rana. (Green.) (Fig. 

 256.) (Pal. N. Y., Vol. VII., p. 

 19, Pis. VII., VIII., VIII.A.) 



Distinguishing Ch aracte rs. — 

 Sub-semi-circular outline of 

 cephalon ; sub-pentagonal outline „ „„. D , 



r 'in Fig. 256. Phacops rana. A perfect 



Of glabella, the posterior furrOW specimen (after Hall and Clarke). 



alone distinguishable; prominent, grooved doublure; eyes 

 with forty to fifty lenses; axis of thorax widest about 



