FOSSILS 01'' THE LOWER SILURIAN. 239 



in many individuals, very depressed-convex, or quite flattened, as occvin-ing 

 on the surface of the shale in which they are imbedded; glabella distinctly 

 conical, moderately tapering above the occipital furrow, and broadly 

 rounded in front; marked by three pairs of distinct transverse furrows, which 

 are directed obliquely backward from their outer ends; the posterior pair 

 almost or quite meeting in the middle, the others shorter, and situated at 

 almost equal distances from each other. Occipital furrow well marked, 

 proportionally wide and shallow; occipital ring narrow, not well defined. 



Fixed cheeks very broad, nearly two-thirds as wide as the glabella, 

 depressed-convex; frontal limb short, the border and inner part of nearly 

 the same width; sides of the limb in front, wide, and slightly rounded at the 

 antero-lateral angles; posterior limb wide at its junction with the glabella, 

 and rapidly narrowing outward, being about once and a half as long as its 

 greatest width; ocular ridges slender, but very distinct; rising from the 

 anterior angle of the eye and uniting with the glabella near the anterior 

 furrow, forming a slightly curved line parallel with the marginal furrow of 

 the head. 



Facial sutures directed inward from the anterior margin of the head to 

 the eye-lobe, behind which they are directed outward and backward to the 

 posterior margin of the head, at an angle of about forty degrees to the 

 occipital line. 



A form of movable cheek found associated in considerable numbers 

 with the glabellas, and corresponding in size and character, is narrowly 

 triangular, the posterior extremity terminating in a short, blunt spine, 

 slightly curved; inner angle strongly notched for the reception of the eye- 

 lobe, and the outer margin bordered by a thickened, rounded rim, which 

 gradually increases in width to the base of the spine. The facial suture 

 corresponds to the margin of the fixed cheek above described, and, on the 

 under side, the anterior border is prolonged in the form of an acute process, 

 to extend along the anterior border of the frontal limb. 



The pygidium associated with the above specimens is minute, trans- 

 versely subelUptical in form, most strongly rounded on the front border, 

 with a wide axis terminating obtusely a little within the- posterior margin. 

 The axis is marked by five rings, exclusive of the terminal ones. Lateral 



