322 



NORTH AMERICAN INDEX FOSSILS. 



furrows indistinct. Eyes large, conspicuous, bearing numerous 

 lenses. Thorax of ii segments, with grooved pleura which are 

 rounded at their extremities. Pygidium semicircular, moderately 

 large, composed of few annulations; margin entire. Siluric- 

 Upper Devonic. 



170. P. pulchella Foerste. * Siluric. 



Glabella strongly convex laterally; its sides forming an angle 

 of about 50° with each other. Fixed cheeks strongly convex and 

 sharply defined posteriorly. Length of glabella and occipital ring 

 5.2 mm., equalling the width of the anterior portion of the glabella. 

 Neck furrow deepest laterally. 



Clinton of Ohio and Tennessee. 



171. P. logani Hall. (Fig. 1636.) 



Devonic. 



Fig. 1636. Phacops logani. (After Hall.) 



Differs from P. rana in the presence of two weak anterior pairs 

 of glabellar furrows besides the strong posterior pair; eyes with 

 about 100 lenses; a prominent tubercle marks the ends of each 

 annulation of the thoracic axis and of the posterior glabellar lobe; 

 the pleural segments of the pygidium are bent backward more 

 strongly and are grooved medially. 



Helderbergian of New York, New Jersey and Oklahoma. 



172. P. cristata Hall. (Fig. 1637.) Devonic. 



Cephalon with genal spines ; a strong furrow beneath the margin 

 of the cephalon bears laterally ten or eleven crenulations. Gla- 

 bella strongly projecting. The two anterior pairs of glabellar fur- 

 rows indistinct; the posterior pair deep but undefined, making the 

 posterior lobes obscure. Center of neck ring and each annulation 



