494- SILURIAN TRILOBITES, 



genus; the>4iumber of vertical rows of lenses in each eye is 

 seventeen, and the greatest number of lenses in a row is 

 five^ and this number only in a few rows, the other rows having 

 four, three, and two; lenses prominent and not closely packed, 

 cups proportionately small, attachment processes visible, cornea 

 present as partitions between the oblique rows; as far as we 

 have been able to observe, the number of lenses in each eye is 73. 



Thorax — Length about equal to width; axis very prominent and 

 semitubular, width throughout almost the same, and equal to that 

 of the side lobes: fore rings arched forward, outer ends strongly 

 nodular; lateral lobes horizontal between the axial grooves and 

 the fulcra; at fulcra deflected at an angle of GS^'-TO", width of 

 horizontal portion about two-thirds that of the deflected portion; 

 pleural furrows deep, vanishing about midway between fulcra ends 

 in decorticated specimens; pleurae recurved, facets large and 

 procurved, anterior ridges triangular, posterior ones robust and 

 merging into the facets. 



Pygidium. — About twice as wide as long, subtriangular, with 

 a slight transverse central arch ; axis conspicuous, slightly 

 depressed between the side lobes, eight rings present; anterior 

 ones intensely arched with a forward inclination, posteriorly 

 diminishing in this respect until the terminal piece almost merges 

 into the border, posterior width a little less than half of the 

 anterior width; six to seven very distinct pleurse on each side, 

 very convex, steeply depressed at the sides; pleural furrows deep 

 and wide, terminating at the borders; interpleural sutures distinct; 

 axial furrows distinct; the whole surface of the decorticated 

 specimens shows indication of strong granulation. 



Qhs. — Owing to the great tumidity and rugosity of the glabella, 

 the deep slit-like character of the glabella grooves (overhung in 

 the case of the first pair by the frontal lobes) in the large speci- 

 mens of this form, we were disposed to make a separate species 

 of this type; but after an inspection of a large number of speci- 

 mens we conclude that this greater tumidity, etc., results from age. 



We have not seen a complete thorax nor the latter with a 

 pygidium attached. 



