FOSSILS OF THE POTSDAM GEOUP. 209 



shell, while in others there may be fifteen or twenty. Concentric lines of 

 varying strength cross the radiating lines at irregular distances. 



The shell is a very pretty and strongly-marked form, and not easily 

 confounded with any previously-known species.* 



Formation and locality. — In sandy shales of Lower Silurian age, proba- 

 bly of the horizon of the Potsdam or Calciferous, at Eureka, Nevada. 

 Collected by J. E. Clayton, esq. 



CEUSTACEA. 



Genus CONOCEPHALITES Zenker, :=CONOCORYPHE Corda. - a. 



- Subgenus CEEPICEPHALUS Owen, % ^ LOGANELLUS Devine. 



The following species of Trilobites possess some peculiar features, 

 which, being common to the whole, mark them at once as a distinct generic 

 type or group. In many of these features, they closely resemble some of 

 the forms described under the name Conoceplialites^ from the same horizon 

 in Wisconsin; while at the same time these peculiar characters distinguish 

 them from the greater number of those species. They all possess more or 

 less distinctly the ^'' slipper-shaped^^ glabella referred to by Dr. D. D. Owen, 

 in his generic description of Crepicephalus, and all appear to have been more 

 or less distinctly marked by three pairs of glabellar furrows, although some 

 of them so faintly as to be seen only by the reflection of light across their 

 surfaces. Another marked peculiarity is the great breadth of the frontal 

 limb between the facial sutures along the anterior border, most of them 

 widening perceptibly in front of the eyes to the anterior margin of the 

 head, where the rim is intersected, almost at right angles with the border, 

 by the suture-line. The great width of the fixed cheeks opposite the eye 

 is another peculiar character of the entire group, in several cases exceeding 

 one-half the width of the anterior end of the glabella; while in only one 

 example, C. (C.)unisulcatus, is this feature reversed, and that to only a limited 

 degree. 



Among the Wisconsin species, the frontal limb is usually not wider 



* Orthis Barabouensis Winchell, from the Potsdam sandstone near Spirit Lake, 

 Wisconsin, sijecimens of which we have lately examined, is closely -related to this 

 shell, but less strongly plicated and more deeply sulcated. — K. P. W. 

 14 p B 



a^-~ c 



