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. 



Fyyidium. — Distinctly triangular, proportion of width 

 to length (leaving out the terminal spine) is about as 

 three to two, moderately arched; axis ver}^ distinct, a few 

 of the anterior segments arched forward centrally, and in 

 decorticated sjDecimens the same segments are provided with sub- 

 triangular articular spaces; normal number of rings in fully 

 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 A^arying length in different individuals; outer ends of 

 the anterior segments bear large nodes bounded on their inner 

 sides b}^ 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 pleurse, 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. 



01 )s. — 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 

 tojD and l^ottom, 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 



