Monograph of the Crustacea of the Cincinnati Group. 119 



3-100 to 4-100 in its greatest breadth, in the largest specimens ; valves 

 obliquely subreniform, broadest near the anterior end, ^vith a straiglit 

 hinge line, which is a little shorter than the greatest length of the 

 valve ; anterior end projecting beyond the hinge ; center moderately 

 convex with a proportionally broad, deep channel, just within the mar- 

 gin, extending all around it, except for a short distance at the poste- 

 rior extremity, near the dorsal margin. The. body of the valve is 

 crossed obliquely by two broad, deep furrows, having their origin on 

 the dorsal margin, the posterior one, situated a little more than one- 

 third of the length of the valve from the posterior extremity, and ex- 

 tending fully two thirds across it ; the anterior furrow is situated just 

 behind the anterior third of the length, and its lower portion is more 

 strongly curved forward than the other. Eye tubercle large, pedun- 

 culated, very prominent, and spreading at the top, its surface equal to 

 about one third the width of the valve, and its height at the posterior 

 margin equal to the breadth at the top, while the interior margin is 

 ):)ut little elevated, giving an obliquely sloping, circular surface, with 

 denticulated border. This surface, under a strong magnifier, is seen 

 to l^e covered by five eye-like facets, similar to those of the eyes of 

 trilobites of the genus Illa^nus. 



Found abundant in the rocks of Mount Auburn, 390 to 440 feet 

 above low water-mark, and at about the same elevation on all the hills 

 surrounding Cincinnati. Rarely, if ever, seen below 300 feet above 

 low Avater-mark, but extending from its commencement to near the 

 Upper Silurian. It is a common fossil. 



Beyrichia Tumifrons — (Hall, 1871). 



Carapace small, subreniform or semi-elliptical, dorsal margin straight, 

 nearly as long as the entire length of the valve ; anterior and posterior 

 extremities equal in width, or sometimes having the anterior a little 

 wider ; extremities sharply rounded, basal margin very broadly 

 rounded. Surface of valves moderately convex, with a deep, narrow 

 marginal groove on the basal margin, which becomes obsolete on the 

 ends before reaching the dorsal angles, leaving a shar]) carinate border. 

 Body of the valve strongly constricted by two deep, oblique sulci, the 

 posterior one Originating in the basal gi'oove, and extending obliquely 

 backward two thirds across the valve ; the second extends entirely 

 across the valve, at about one third the length from the anterior end, and 

 is strongly curved backward in the middle, so that the anterior portion 

 of the valve forms a rounded area, which is often the most prominent 

 part of the body, though in some specimens the central ridge is equally 



