506 
SILURIAN TRILOBITES, 
between the fulcra and outer ends, obliterating the pleural grooves 
as above stated, outer ends flattened and claw-shaped, the latter 
feature very distinct on the posterior pleurae; sutures very dis- 
tinct; evidences of granulation very small; axial grooves mode- 
rately distinct. 
Pygidium. — Distinctly triangular, proportion of width 
to length (leaving out the terminal spine) is about as 
three to two, moderately arched; axis very distinct, a few 
of the anterior segments arched forward centrally, and in 
decorticated specimens the same segments are provided with sub- 
triangular articular spaces; normal number of rings in fulty 
developed specimens is sixteen to eighteen, and even attaining 
nineteen exclusive of the terminal appendage, no distinct traces 
of tubercles observed, sides straight, tapering gradually and in 
perfect specimens being inseparable from the produced spine, 
which is of varying length in different individuals; outer ends of 
the anterior segments bear large nodes bounded on their inner 
sides by slight depressions similar to those referred to on the 
thoracic segments; in some cases posteriorly depressed between 
the side lobes; axial grooves distinct, anchylosing margins of the 
side lobes straight between the fulcra, ends sharply recurved; in 
their inflation and deflection agree with the lobes of the thorax, 
but in some cases the slope is gradual from the axial grooves; 
eleven pleurae, each succeeding one having a greater backward 
curve until the last is nearly parallel with the axis; pleural grooves 
wide and shallow; anterior ridges very distinct, thickening at the 
inner edge of the limb, and thence vanishing towards the outer 
edge, and forming a tuberculous ridge along the inner edge of the 
limb; sutures distinct; limb wide, its inner edge distinctly marked 
by a suture; to this limb the tail spine is attached. 
Obs. — The rows containing the maximum number of lenses are 
alternately one lens higher and lower top and bottom than the 
preceding row, while the succeeding rows on each side are level 
top and bottom, and in this manner each of these rows fore and 
aft of the central portion loses a lens until the corner rows have 
the minimum of two to three lenses. The maximum of eleven 
