CRUSTACEA. 35 



Pygidium sub-ovate-triangular in outline, posteriorly truncate or broadly emar- 

 ginate. Surface depressed-convex, with a general upward inflection poste- 

 riorly. Margin thickened, narrow, widening into the post-axial border, 



Axis convex, width equal to one-fifth the width of the shield along a given 

 annulation, regularly tapering to obsolescence upon the posterior margin ; 

 composed of sixteen to twenty transverse, somewhat flattened annulations, 

 separated by moderately strong furrows. 



Pleura wide, convex near the axis, thence regularly depressed toward the 

 margin; composed of sixteen to eighteen broad, slightly flattened annula- 

 tions, which become obsolete upon the margin. These are faintly sulcate, 

 and are separated from one another by furrows, which are deepest and 

 broadest near the margin. The border is ornamented by a fringe of conspic- 

 uous spines, one in continuation of each pleural annulation. Of these spines 

 the anterior pair is the shortest, and their length increases backward to the 

 ninth or tenth pair, thence becoming shorter to the terminal pair, in which 

 the spines are broader and stouter than elsewhere, and distinctly curved 

 upward and toward the axial line. These terminal spines are separated, 

 at their bases, by a distance equal to the width of the axis on its anterior 

 margin The surface of the pygidium is covered with regularly arranged 

 tubercles, of which the axis bears five longitudinal rows, one axial and four 

 lateral (becoming two on the posterior annulations), each annulation bearing 

 a single row of three or five tubercles Each pleural annulation bears two 

 rows, one for each anterior and posterior limb. Upon the lateral margins 

 the tubercles become stronger and are more scattered, while the posterior 

 border between the terminal spines is smooth. In the New York specimens 

 the entire surface ornamentation, with the exception of the marginal spines, 

 is frequently almost or quite obsolete. 



Dimensions. An average pygidium from the Falls of the Ohio measures 60 

 mm. in length and 79 mm. in width ; one from western New York, 46 mm. 

 in length and 67 mm. in width. The largest specimen observed has a length 

 of 67 mm. and a width of 100 mm.; the smallest a length of 35 mm. and a 

 width of 51 mm. 



