132 Monogra'pli of the Cnistacea of the Cincinnati Group. 



iug a riclge that extends obliquely backward and outward from the an- 

 terior segment of the mesial lobe, across each, to the posterior lateral 

 margins, where these ridges terminate in prominent, rounded, diverg- 

 ing spines ; while the posterior lateral margins between these spines 

 and the lateral angles are each armed with four smaller, slender 

 spines, directed obliquely backward and outward. Four similar spines 

 also occupy the truncated middle parts of the posterior margin between 

 the two larger ones ; surface smooth, excepting a few very minute, 

 scattering asperities on the spines. 



The pleurse of the posterior thoracic segment are smooth, and have 

 each a strong mesial ridge extending straight outward to the lateral 

 extremity, where it curves abruptly backward and is produced into a 

 long, sharp spine, extending as far backward as the longest spines of 

 the pygidiuni, or farther. 



Length of pygidium, exclusive of spines, 0.19 inch; breadth, 0.55 inch. 

 Transverse diameter of first thoracic segment in advance of pygidium 

 0.70 inch, length of each pleurte 0.23 inch; anterio-posterior diameter 

 of the same 0.08 inch, length of larger lateral spine of each 0.38 inch. 



Fragments of this species are found at all elevations, from low water- 

 mark to the top of the hills back of the city, but they are most abun- 

 dant in the lower 200 feet of the rocks. The specimen belonging to 

 Dr. Hill was found in the marl in Eden Park, less than 200 feet above 

 low water-mark. 



Genus Ceraurus — (Green, 1832) as redefined by McCoy. 



Cephalic shield granulated, semi-circular, lateral angles prolonged in- 

 to spines ; glabella clavate, reaching the front margin, gibbous, with 

 three subequal, segmental furrows on each side, the basal one retro- 

 flexed ; eyes, small in the midst of the cheeks ; eye line going from 

 their base directly to the outer margin, which they cut considerably in 

 front of the angles ; thorax of eleven segments ; plenne wider than the 

 axis, each having its origin thickened into a large, oblong tubercle, 

 cleft by a deep, diagonal, pleural sulcus ; a little beyond this a small, 

 rounded tubercle, beyond which the extremity is flat and falcately 

 pointed ; pygidium with a short, four-jointed axis, and the margin pro. 

 duced into three strong spines- on each side, the anterior largest. 



Chirurus of Beyrich is merely a synonym, and should therefore be 

 suppressed. 



Ceraurus pleurexanthemus — (Green, 1832). 

 Cephalic shield crescentiform, with a prominent connate articulation 



