LOWER HELDERBERG ROCKS. 
353 
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. 
Pliacops logani (n. s.). 
Plate LXXIII. 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 more 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. 
[ Paleontology III.] 45 
