Monograph of the Crustacea of the Cincinnati Group. 137 



tei'ior part of the foot, as the animal pressed forward upon the an- 

 terior part, drew the mud up after it, leaving a correct impression 

 of the anterior part, and an elevated representation of the posterior 

 part. 



The specimens were found by C. B. Dyer, Esq., on shale, from an 

 excavation on the west side of the Walker Mill road, in Cincinnati, 

 about 100 feet above low water-mark, and are now in his collection. 

 I do not know that any other person has been so observing as to find 

 them. 



Subgenus Isotelus — (DeKay, 1824), as redefined by McCoy. 



Large elliptical; huckler semi-elliptical, with the angles rounded, or 

 produced backward into spines ; Glalella indistinctly defined ; eye 

 lines meeting at an acute angle at the front margin, thence diverging 

 backward, slightly approximating again about the middle of their 

 length, Avhere the " hiant" eyes are situated, and again diverging to cut 

 the posterior margin, near the angles ; thorax of eight segments, axis as 

 ■wide or wider than the lateral lobe, the ends of the pleural rounded 

 with a strongly marked triangular facet and pleural groove ; pygidium 

 resembling the buckler in size and shape, trilobed and generally with a 

 broad, smooth margin ; axis and lateral lobes with fine segmental fur- 

 rows or smooth. 



Asaphus (subgenus Isotelus) megistos — (Locke, 1842). 



The cephalic shield is anteriorly nearly pei-fectly elliptical, broadly 

 and thinly margined, posteriorly arcuate, and terminated at the angles 

 by spines or pointed processes, extending backward, beyoiid the two 

 first thoracic segments. The eyes are prominent, large, furnished ex- 

 teriorly, each, with a crescent-shaped cornea, and j^laced rather nearer 

 to the posterior edge than to the outer margin of the shield. From 

 the corner of each eye a sutural line extends forward, meeting at the 

 anterior margin of the shield, and inclosing a lozenge-shaped, leaf-like 

 frontal space. A thorax, with eight segments, middle lobe, sub-cylin- 

 drical ; lateral lobes somewhat flattened, and descending abruptly at 

 their lateral extremities, which are broad, flat, and rounded beneath, 

 and admirably fitted to sliding over each other when the animal should 

 contract or roll himself, according to a well known habit of the genus. 

 Pygidium posteriorly elliptical, anteriorly circularly arcuate ; length, 

 horizontally, less than two thirds of the width, having two obscure, 

 longitudinal depressions, continuous with the abdominal furrows, and 



