146 . Mojiograph of the Crustacea of the Cincinnati Group. 



mesial one, and with their flattened margin leather more than one tliird 

 the breadth of the anterior end of each, and each showing obscure 

 traces of six or seven furrowed segments. 



Entire surface smooth. Length of a specimen, 0.54 inch ; breadth 

 at the widest point across the posterior part of the head, 0.25 inch ; 

 length of head, 0.27 inch ; length of pygidium, 0.11 inch. 



The specimen belonging to Dr. H. H. Hill was found in the excava- 

 tion for Gilbert avenue, in Eden park, about 180 feet above low-water 

 mark. 



Genus Triarihrm — (Green, 1832). 



General form, an elongated ellipse, rather pointed at the tail ; cephalic 

 shield somewhat semi-circular, deeply trilobate, rounded off laterally 

 without projecting spines ; glabella of equal width from base to front, 

 trilobed on each side, rounded and convex in front, neck segment dis- 

 tinct. Thorax with thirteen segments, middle and lateral lobes of 

 equal width, axial lobe convex, with a central row of minute spines ; 

 pygidium obtusely semi-circular, witli six or seven segments. 



Tlie name was from the three articulations in the glabella, then sup- 

 posed to be the three articulations of the abdomen. 



Triarthriis Beckl — (Green, 1832). 



t 



General form, an elongated ellipse, narrower at the posterior extrem- 

 ity, sides somewhat straight ; cephalic shield broadly semi-oval, rounded 

 on the margin posteriorly ; glabella of equal width from base to front, 

 rounded and convex in front and deeply trilobate on each side, prominent 

 thoracic ring at the base, having a minute spine on the middle of the cen- 

 tral lobe ; side lobes narrowing and pointed in front ; eyes indistinct, 

 situate at the point of union between the side lobes and the margin an- 

 terior to the third lobe of the glabella ; thorax with thirteen segments, 

 lobes of equal width, and narrowing toward the pygidium ; axial lobe 

 convex, and having a short spine or prominent tubercle on the back of 

 each segment, forming a row of spines from the neck segment to the 

 pygidium ; lateral lobes somewhat flattened, each segment deeply 

 grooved along the middle, and obtusely pointed ; pygidium with seven 

 segments in the middle lobe and five in the lateral lobes ; posterior ex- 

 tremity obtuse. It varies in size from half an inch to one and a half 

 jiiches in length. 



In Mr. Dyer's collection there are some very good specimens. 



