lo8 Monograpli of the Crustacea of the Cincinnati Group. 



converging toward an obscure posterior tubercle. The anterior outline 

 of the pygidium exhibits three slight lobes (corresponding with those 

 of the thorax), the two exterior of which are very distinctly marked 

 by a transverse depression. 



When the posterior shell of the tail is decorticated, an interior shell 

 is exposed, which forms all around a deep trough, or "cavetto," beauti- 

 fully marked with a "venalian" of eccentric curved and branched lines. 



The above-named jDosterior tubercle is very nearly the focus of the 

 elliptic outline of the pygidium, is just anterior to the marginal cavetto, 

 and is the center around which the curved lines originate, each passing 

 a little further back than the other, and advancing outward and for- 

 ward, until they successively disappear on the anterior margin of the 

 cavetto. 



It is distinguished from the A. gigas by the aculeate processes or 

 spines, by the perfectly elliptic terminations, by the simple (not raised) 

 margin of the shield, and by the proportions of the pygidium, the 

 gigas having the length four fifths, and the megistos three fourths only of 

 the width. The latter is also much more prominent than the former, 

 and the pijgidium and sides much more abrupt in their descent. 



Fi-agments are abundant throughout the blue limestone, from low 

 Avater-mark in the Ohio to at or near the Upper Silurian, but a good 

 specimen is extremely rare. 



The fragments indicate that it sometimes reached 20 inches in length, 

 and near 12 inches in breadth. The best specimens' found do not gen- 

 erally exceed 3 or 4 inches in length. No locality can be pointed out 

 at which a reasonably good specimen would be certain to reward a three 

 days' search by a collector. 



Asaphus (subgenus Isotelus) gigas — (DeKay, 1824), 



General figure oval oblong, with the sides rather straight, and entire^ 

 surface finely punctate ; head rej)resenting a spherical triangle, obtuse, 

 or more or less rounded at the posterior extremities, convex, descend- 

 ing from between the eyes to the anterior border, which has a narrow, 

 raised rim ; eyes elevated, prominent, subpedunculated, strongly sup- 

 ported on the inner and concave side by a projection of the glabella ; 

 cornea oblong, lunated, highly polished ; facial suture continuing from 

 the center of the front, nearly parallel to the margin, until in a line 

 with the eye, when it turns backward, and leaving the eye upon the 

 maxillary portion, turns outward and backward, coming out at the 

 base of the shield distant from the angle ; thorax with eight articula- 

 tions, the middle lobe about once and a half the breadth of the lateral 



