LOWER HELDERBERG ROCKS. 338 



Fig. 9. A fragment of the thorax of a specimen of medium size. 



Fig. 10. The pygidium of a small specimen. 



Fig. 11. Profile view of the same. 



Fig. 12. The pygidium of a large individual. 



Fig. 13. An enlargement of the surface. 



Geological position and locality . In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains, Albany county, and in Herkimer county. 



Phacops Ionian! ( n. s.). 



Plate LXXIIL Fig. 15 - 25, 



General form elliptical. Head semicircular in outline, broadly concave 

 above, with the posterior angles curved and declining more abruptly. 

 Glabella somewhat pentagonal ; length and greatest breadth nearly in 

 the proportion of three to four ; very prominent in front, projecting 

 beyond the rudimentary frontal limb, which becomes naore developed 

 on each side, and below which is a defined groove, marking the limits 

 of the lower side of the cephalic test. Upper surface convex, gibbous 

 in front, having two pairs of transverse grooves, the middle and posterior 

 ones of which are but faintly defined ; while the third or anterior ones, 

 extending from the inner angle of the eye backwards and a little in^^ 

 wards, are scarcely conspicuous, and, on many specimens, not observable. 

 First annular furrow strongly defined, and sometimes with a small tu- 

 bercle at the summit : first or intercalated annulation narrow and well 

 defined, and terminated at each extremity by a strong oblong tubercle, 

 which is wider than the annulation, and usually marked by two pus- 

 tules at the summit, one on each side of the centre. Occipital furrow 

 wider and more strongly defined than the intercalated one, slightly 

 sinuous, and terminating in a deep cavity at each extremity : occipital 

 annulation broad and strong, slightly sinuate at the extremities, and, 

 when entire, marked by one larger central pustule and several smaller 

 ones. 



[ PAL-T.ONTOLOaY IIL] 45 



