256 THE CHICAGO ACADEMY OF SCIENCES. 



curves rather abruptly inward, to meet the occipital furrow; 

 this furrow is most strongly impressed posteriorly, but is fainter 

 even than the dorsal furrow. The outer portion of the cheek' 

 is sub-lunate in outline and descends abruptly to the lateral 

 margin, anteriorly it is concave below the ocular ridge, posteriorly 

 its upper margin is elevated into a depressed ridge which is 

 broader but less sharply denned than the ocular ridge, becom- 

 ing more elevated as it approaches the base Of the cheek spine; 

 below this ridge the surface is concave, the lateral and pos- 

 terior margins are raised into an elevated, rounded border, the 

 posterior extremity is produced into the slender cheek spines 

 which extend obliquely -Upward and outward. Occipital fur- 

 row broad and shallow across the median lobe of the glabella, 

 becoming narrow and deeply impressed back of the posterior 

 lateral lobes of the glabella. Median portion of the occipital 

 segment broad and convex, bearing two slender, nearly erect 

 and slightly diverging spines whose bases are widely separated ; 

 on the median line between the spine bases and situated a 

 little in front of them is a strong rounded tubercle ; the occipital 

 lobes are strongly convex and rather sharply separated from 

 the median portion, they are sub-oval in outline, with their 

 longer axes directed obliquely backward and outward. Thorax 

 and pygidium unknown. 



The dimensions of a nearly perfect cephalon are: width 

 47 mm., length 17 mm., convexity 22.5 mm., width of glabella 

 26.5 mm., distance between eyes 35 mm., distance between 

 bases of cheek spines 42.5 mm. 



Remarks. The specimen of this species here illustrated 

 is from the Van Home Collection in the Walker Museum, and 

 is the most complete cephalon which has been observed. The 

 species has no nea^ relative in other Silurian faunas of America, 

 except C. depauperata van Ing., which is a much smaller form; 

 it is somewhat closely allied to the Bohemian species C.'ver- 

 neuili (Barr.), but among other characters it lacks the mar- 

 ginal spines upon the cephalon which are present in that species. 

 The name originally applied to the species by Warder was long 

 overlooked and it was redescribed by Hall as Acidaspis dance, 

 and by Winchell and Marcy as Acidaspis ida, the type speci- 

 mens of both these authors being from Bridgeport, Illinois, while 

 Warder's specimens were from near Springfield, Ohio. Dr. 

 J. M. Clarke has shown that both these latter names are synony- 

 mous with Warder's earlier name. 



Localities. Bridgeport and Hawthorn. 



