CRUSTACEA. 
3 
along the axial line. Upon the pleurae the segments are.grooved, the ante¬ 
rior limb of each being small and becoming obsolete on the beveled planes 
of articulation. 
Pygidium spherically sub-triangular, truncate and strongly emarginate on the 
posterior border. Length to width as 1 to 1.3. 
Axis broad, rapidly narrowing and ending abruptly in a blunt elevation 
within the posterior border; its terminal portion is flattened above, and 
from its apex a low ridge passes over the post-axial area. Axial annula- 
tions six. 
Ple.um broad, and abruptly deflected. Anterior margins strongly beveled 
by the articulating planes, and meeting the lateral margins at an angle of 
ninety degrees. Pleural annulations four, flattened near the margin and 
sometimes showing a tendency to bifurcate. 
N 
Hypostoma sub-quadrate; lateral margins strongly incurved, posterior border 
emarginate. Anterior angles auriculate, margins thickened and slightly 
reflexed; centrum elevated, sharply carinate. 
Surface Ornamentation. The cast of the external surface of a nearly entire 
individual, which served as Dr. Green’s type of the species, shows faint 
tubercles over the glabella and free cheeks, but these appear to be obsolete 
upon the occipital annulation and the rest of the body. 
Dimensions. The smallest individual of this species which has been 
observed is the type specimen. It affords the following measurements; 
Length 63 mm.; width 45 mm. The largest individual observed measures ; 
Body. Cephalon. Thorax. Pygidium. 
Length_ 130 mm. 30 mm. 75 mm. 25 mm. 
Width_ 75 mm. 75 mm. 70 mm. 30 mm. 
Fragments of still larger individuals are sometimes found, a single pygidium 
from the Upper Helderberg limestone of the Falls of the Ohio measuring 
45 mm. in length and 35 mm. in width, indicating an individual of quite 
