128 Monograph of the Crustacea of the Cincinnati 'Group. 



glabella clavate, narrow behind, arched and expanded in front, ex- 

 tending beyond the center of the inner lateral lobe ; pygidium com- 

 posed of two or three articnlations above, with a fonrth and last, which 

 is very long, and abruptly narrowing toward the extremity ; lateral 

 lobes with three articulations on each side, which are marked by a nar- 

 row groove for about one third of their length ; central lobe separated 

 from the lateral lobe by a deep, narrow groove." 



This species is found very rarely about Cincinnati at an elevation of 

 about 400 feet above low \Yater-mark. A. good specimen of either the 

 cephalic shield, thorax, or pygidium is regarded as very rare. Its 

 range is not known. 



Fourth Subfamily Paradoxince. 



Pleura, flat, horizontal, without facets, with a straight pleural 

 groove, and terminating in spines inclining backward. 



This subfamily is composed of a group of long-bodied trilobites, 

 having for the most part large, cephalic shields, with the angles pro- 

 longed into spines, and very small, flat, caudal shields of few segments, 

 none of them, I (McCoy) believe, can roll into a ball ; their pleura are 

 very wide and horizontal, not bent downward at the ends, so that the 

 body is flat ; they are destitute of facets, and havealong, straight, pleural 

 groove extending their whole length, dividing each into two tumid 

 portions, one or both of which are extended into long, sharp spines in- 

 clining obliquely backward. This latter character, of all the pleurpe 

 terminating in separated spines, gives a peculiar physiognomy to the 

 trilobites of this group. The thoracic segments vary from eight to 

 twenty-three in number. 



This subfamily includes the genera Acidasjns, Ceratiriis, etc. 



Genus Acidaspis — (Murchison, 1839). 



" Cephalic shield wider than long, subquadrate or semi-circular, 

 margin tumid, often spinous, lateral angles forming long, slender 

 S2oines ; glabella convex, narrow in front, not reaching the front margin, 

 with two or three lateral lobes on each side, base prolonged backward 

 into one or two long, thick spines ; antennary pores usually distinct, 

 close to the front of the glabella ; eye line tumid, cutting the margin 

 on each side in front of the glabella, and extending from the eyes, 

 which are very small, to the posterior margin rather nearer the angles 

 than the glabella on each side ; thorax of eight joints, axis convex, 

 narrow ; pleurce wide, perfectly flat, each with a nearly mesial pleural 



