FOSSILS OF THE POTSDAM GROUP. 211 



Cephalic shield semicircular forward of tlie occipital line, the posterior 

 extreniities extending in ratlier broad spines, which reach to the fifth or 

 sixtli thoracic segment, and when in their normal condition are nearly 

 parallel to the axial lobe, but are usually sjnead out laterally by the flat- 

 tening of the head. 



Glabella rather small, rounded-conical, narrowing gently from the 

 occipital furrow forward; front regularly rounded; surface depressed-con- 

 vex, marked by three pairs of transverse furrows, which extend rather more 

 than one-third of the distance across the glabella, and are strongly curved 

 backward at their inner ends; occipital furrow broad and shallow; occi- 

 pital ring narrow. Fixed cheeks narrow at the eye, separated from the 

 glabella by rather distinct furrows. Frontal limb long, about half as long 

 as the glabella, strongly and nearly equally concavo-convex; posterior 

 limb narrowly triangular, reaching about two-thirds of the width of the 

 movable cheeks. 



Movable cheeks of moderate size, irregularly triangular, distinctly 

 notched at the inner angle for the reception of the eye-lobe; surface con- 

 vex, irregularly striated by very fine lines, which radiate from the base of 

 the eye-tubercle; marginal rim of moderate width, flattened, the flattening 

 extending upon the spine ; occipital furrow well pronounced and reaching 

 to the inner margin of the posterior spine. 



Suture-lines distant, cutting the anterior margin of the head with a 

 slightly inward curvature, leaving Jie frontal limb nearly half as wide as 

 the entire width of the base of the head ; thence directed gently inward, 

 with a slight curvature to the anterior angle of the eye, and, after passing 

 around the eye-tubercle is directed in a straight line to the posterior mar- 

 gin of the head, which it reaches at about two-thirds of the distance between 

 the glabella and outer margin of the cheek. 



Fye distinct, slightly elevated, distinctly reniform, and about one-thii-d 

 as long as the glabella and occipital ring. 



Thorax considerably wider than long, distinctly trilobed, rather rapidly 

 tapering from the third or fourth segment j^osteriorly, and composed of 

 twelve segments. Axial lobe rather narrow and gradually tapering through- 

 out, scarcely equaling at its widest part one-fourth of the entire width of 



