498 PALAEONTOLOGY OF ILLINOIS. 



with the exposed surfaces seen in a rolled-up specimen, narrowing off 

 laterally very rapidly with a strong backward curve ; all more or less 

 angular at the extremity, the posterior ones being rather pointed; lap- 

 ping surfaces apparently wide. 



Pygidium subtrigonal, and of near the same size as the head, entirely 

 without any indications of trilobation or segments. 



Whole surface smooth, excepting a minute pitting, most distinct on 

 the movable cheeks. 



Length (measuring over the curve of a rolled up specimen) 2.75 inches, 

 breadth 1.30 inch, length of head at the middle about 0.75 inch, breadth 

 between the eyes 0A7 inch, hight of eyes on the outer side 0.20 inch. 

 Breadth of axial lobe of thorax 0.85 inch, antero-posterior diameter of 

 each of the first four or five segments of same near the middle 0.15 inch. 



This species seems not to be nearly related to any of the described 

 species with which we are acquainted. Its most marked characters are 

 the prominence of its eyes, and the almost entire absence of any traces 

 of trilobation in its thorax and pygidium, as well as the great breadth 

 of the mesial lobe of the same, as indicated by a very obscure depres- 

 sion, and a minute projection on the anterior margin of each thoracic 

 segment, on a line nearly behind the outer edge of each eye. These 

 little projections do not extend upward, but forward, and fit into corre- 

 sponding notches in the posterior margin of each succeeding segment 

 in front. As the margin of its head and the posterior edge of its pygid- 

 ium are iu the specimens more or less imperfect, we cannot determine 

 exactly their outlines. 



In some respects this species resembles the young individuals of 

 Isotelus megistos, of Lock, though it differs in not having its cheeks 

 produced into pointed terminations behind, while its eyes are more 

 prominent and situated farther forward, and the mesial lobes of its 

 thorax much less defined and distinctly wider. Its pleurae also differ 

 in being 4 angular, or a little pointed, instead of rounded at the ends. 



Positiorijnnd locality : Carroll county, near Mount Carroll, and near 

 Oswego^in^Kendall county, in this State, in the Cincinnati shales of the 

 Lower Silurian. 



