296 PALEOZOIC PALEONTOLOGY. 
below. The axis is gradually attenuate, the width at the posterior 
extremity being about one-third as great as at the anterior extremity, 
which is about five-eighths as wide as the greatest width of the lateral 
lobe at its upper margin; its outline is curved and sometimes scarcely 
carinate, the latter feature more often seen in the casts. The number 
of articulations in the axis is seventeen; and on each of the laterai 
lobes are eleven to thirteen ribs, which are little wider than the fur- 
rows which separate them, the whole bending downwards toward the 
outer extremities and uniting in a thickened border. Each rib of 
the lateral lobe is marked by a longitudinal groove, parallel with the 
margin and a little nearer to the upper than the lower margin.”— 
Hall. 
The dimensions of a full-grown pygidium are, approximately: 
length, 53 mm.; breadth, 68 mm. 
Remarks.—Only the pygidium of this species has been detected in 
the New Jersey collections, and the specimens observed are, in almost 
every case, mere fragments. There is no doubt, however, of the 
identity of these imperfect specimens with this rather common species 
of the New York Helderbergian fauna. 
PROETUS PROTUBERANS Hall. 
Plate NSXeNehIS igs 22 
1859. Proetus protuberans Hall, Pal. N. Y., vol. III., p. 351, pl. 73, 
figs. 5-8. 
Only the pygidium of this species has been observed in the fauna 
of the Coeymans limestone in New Jersey. The only specimens 
observed are fragmentary, but they can be distinguished from the 
pygidium of Phacops logani by the flattened marginal border. 
