CRUSTACEA. 9 



animal has been preserved in an extended condition. In casts of the under 

 surface, the usual mode of preservation in the sand} 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 pleura', 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, interior 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 

 pleura. 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 surface of the test on its more prominent 

 portions is marked by the openings of large vertical tubulipores. On casts 



