54 PROCEEDINGS OF THE ACADEMY OF 



rolled-up specimen, showing scarcely any traces of trilobation, and composed 

 of eigbt segments. Mesial lobe, as indicated by very faint impression on each 

 side of the body segment, very wide and depressed, with segments nearly flat. 

 Lateral lobes very narrow, sloping off regularly from the mesial one on each 

 side ; pleura? without furrows, and 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 ; lapping 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 the specimen as rolled together), 2-75 

 inches; breadth, 1-30 inch; length of head at the middle, about 0-75 inch; 

 breadth between the eyes, 0-47 inch ; height of eyes on the outer side, 0-20 

 inch. Breadth of axial lobe of thorax, 085 inch ; antero-posterior diameter 

 of each of the first four or five segmeuts of same near middle, 0-15 inch. 



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

 with which we are acquainted. Its most marked characters are the promi- 

 nence 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 depression, 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 corresponding notches in the posterior margin of each succeeding 

 segment in front. As the anterior margin of its head and the posterior edge 

 of its pygidium are in the specimens more or less imperfect, we cannot deter- 

 mine exactly their outlines. 



In some respects this species resembles young individuals of Isotelus megistos, 

 of Lock, though it differs in not having its cheeks produced into pointed ter- 

 minations behind, while its eyes are more prominent and situated farther for- 

 ward, and the mesial lobes of its thorax much less defined and distinctly 

 wider. Its pleura? also differ in being angular, or a little pointed, instead of 

 rounded at the ends. 



Locality and position. Carrol County and Osage County, Illinois. Cincin- 

 nati group of the Lower Silurian. 



Ill^nus (Bumastds) Graftonensis, M. and W. 



Attaining a rather large size. Head (as determined from internal cast) 

 transversely subelliptic, as seen from above, when placed with the under side 

 on a horizontal plane, its breadth being to its length very nearly as 50 to 30 ; 

 moderately convex, the height being rather distinctly less than half the 

 breadth, and the most prominent part a little behind the middle, while the 

 curve over the middle, from its posterior to its anterior margin, forms about a 

 quarter of a circle. Anterior margin, as seen from above, presenting a nearly 

 transversely semi-elliptic curve, and a subrectangular outline, as seen in a 

 side view ; lateral margins rather narrowly and regularly rounded in outline, 

 into the posterior side. Axial furrows distinct, converging forward to a point 

 nearly opposite the middle of each eye, where they terminate in little flattened 

 oval impressions. Eyes large, forming nearly semi-circular curves, with their 

 posterior ends as near the posterior as to the lateral margins of the head ; 

 each with a broad, very deep, rounded furrow around beneath its outer side, 

 so as to form a kind of obtuse shoulder below, from which the cheeks dropoff 

 nearly vertically, with a slight convexity of outline, to the inferior margins ; 

 palpebral lobes less elevated than the middle of the glabella, and sloping a 

 little outward, with an even convexity over their whole surface; visual surface 

 forming rather narrow convex bands, and showing (in the internal cast) under 



[April, 



