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 live 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, sub- 
semicylindrical, about two-thirds as wide as the contour measure- 
ment of the pleurae, ends of each segment nodular, posterior 
segments distinctly arched forward ; pleurae 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 very robust and continuous, with diminishing 
intensity to the rounded extremities, posterior edges of pleurae 
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 blunt 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 
