490 SILURIAN TRILOBITES, 



angles of the glabella, posteriorly in a line with the basal glabella 

 furrows; curve of lentiferous face front to back semicordioid, 

 apically inclined inward at an angle of about 50'; lenses very 

 convex, the normal number of vertical rows is seventeen, with 

 five lenses in each, except the terminal rows at each end; the first 

 row in front has usually three, the next four, then follow twelve 

 rows of five lenses, their three posterior rows having four, three 

 and two respectively, making a total of seventy-six lenses for each 

 eye, which are separated by minute spaces, but no partitions are 

 visible. 



Thorax — Square, sides almost perpendicular; axis distinct, sul> 

 semic3dindrical, about two-thirds as wide as the contour measure- 

 ment of the pleurie, ends of each segment nodular, posterior 

 segments distinctly arched forward ; pleura? between axial furrow 

 and fulcra horizontal, then intensely deflected, forming almost 

 perpendicular sides, extremities procurved and flattened; pleural 

 furrows distinct on horizontal portions, gradually diminishing on 

 the deflected parts and ceasing about midway between the fulcra 

 and extremities, making the front ridges distinctly triangular, 

 posterior ridges ver}^ robust and continuous, with diminishing 

 intensity to the rounded extremities, posterior edges of pleurse 

 traversed (in decorticated specimens) by a fine groove which shows 

 most distinctly on the deflected portions, evidently marking the 

 thickness of the test. 



Pygidium. — Roughly semicircular, highly convex, anterior 

 margin nearly straight; axis very prominent, composed of eight 

 segments, the first and second segments very distinct, and strongly 

 arched forward; each succeeding segment diminishes in distinct- 

 ness so that the Ijlunt terminal piece is hardly separable from the 

 thickened border, and is half the width of the anterior portion; 

 axial furrows moderately distinct; lateral lobes very tumid, sharply 

 deflected, consisting of six or seven pleurae, furrows of first pair 

 like those of the thoracic pleurae, each succeeding pair becoming 

 fainter till the seventh is rarely discernible and do not reach 

 the edges; sutures distinct; in decorticated specimens a distinct 



