CRUSTACEA. 
9 
animal has been preserved in an extended condition. In casts of the under 
surface, the usual mode of preservation in the sandy shales of the central 
counties where individuals are most abundant, the transverse median groove 
upon the segments appears very broad and deep, and may be misleading in 
determining the true limits of the segments. In such specimens, along 
the low and indistinct longitudinal axial furrows, there are rows of double 
depressions or pits at the anterior and posterior margins of adjacent seg¬ 
ments, which are suggestive of the depressions left by the visceral sup¬ 
ports or basal joints of natatory appendages; but in position they are 
not homologous with those observed in Phacops rana and other species. 
The transverse groove is continued upon the pleurae, passing over the broad 
beveled articulating planes, but becoming obsolete before reaching the mar¬ 
gin. In specimens where the crust is retained, this furrow is visible only as 
a sharply impressed line. 
Pygidium. Outline sub-triangular; length and width equal. Anterior margin 
with a forward curve; lateral margins nearly straight, with an upward curve 
near the posterior extremity, which is produced into a sub-spatulate exten¬ 
sion. Surface convex, faintly trilobate. The angle of convergence of the 
lateral margins is nearly ninety degrees and the anterior basal angles about 
forty-five degrees each. The articulating ring and furrow are conspicuous. 
The axis tapers rapidly, becoming obsolete before reaching the posterior 
margin. Ten annulations may be counted upon it, and eight upon the 
pleum. These annulations are rarely visible upon the dorsal surface except 
in young individuals and become obsolete in mature animals, where traces of 
them can be seen only in casts of the under surface. 
Hypostoma sub-quadrate, auriculate at the anterior angles, emarginate on both 
anterior and posterior margins ; centrum low, convex ; posterior sulcus and 
posterior lateral pits conspicuous ; margins thickened, not reflected. Surface 
pustulose. 
Surface Ornamentation. The surfiice of the test on its more prominent 
portions is marked by the openings of large vertical tubulipores. On casts 
