CRUSTACEA. 
137 
Axis convex, elevated, evenly tapering to an abrupt termination distant 
from the posterior margin; width on the anterior margin about one-third 
the width of the shield ; bearing from eight to twelve narrow annulations, 
which have a slight retral bend along the axial line and appear to be angu- 
lated by the five longitudinal rows of strong tubercles. Of these rows the 
middle one is much the strongest. Upon the elevated termination of the 
axis are a number of small, irregularly scattered pustules. 
PleurcE gently convex for about one-half their width, and depressed upon 
the broad, flattened border; bearing eight annulations, which are grooved 
for their entire length, making the anterior limb of each strong and the 
posterior limb inconspicuous. Each annulation bears upon the anterior 
limb four, five or six strong sub-spiniform tubercles, of which those upon 
the margin are much the strongest and are usually produced into short 
spines. Some specimens bear a terminal spiniform tubercle upon the 
margin between the last two annulations. The pleurm and articulating 
grooves are punctate. 
Doublure broad and flat. 
Dimensions. A thorax from the Hamilton shales measures 6 mm. in length 
and 10 mm. in width; a pygidium from the same formation 6.5 mm. in length 
and 9 mm. in width; a small pygidium from the Upper Helderberg limestone 
3 mm. in length and 5 mm. in width. 
Distribution. Upper Helderberg group. Corniferous limestone; In the de¬ 
composed chert boulders, Canandaigua, Ontario county. Hamilton group; In 
the lower shales at Canandaigua and Fall Brook, Hopewell, Ontario county; 
Eighteen-Mile creek, Erie county. 
Phaethonides cyclurus, n. sp. 
PLATE XXIV, FIG. 26-28; AND PLATE XXV, FIG. 11. 
Cephalon. The intra-sutural portion, which is the only part known, indicates 
a semicircular outline for the head; frontal and lateral areas depressed and 
