42 LOWER CAMBRIAX FOSSILS WALCOTT. 



spine, much as in 0. Itjerulfi ; the spine varies in size and direction, 

 from the young individual, where it is directed backward, to the large 

 adult, in which it extends obliquely outward. The under side of the 

 marginforms a broad " doublure." It is slightly arched downward and 

 narrows towards the postero lateral angles of the head. A slight, 

 curved indentation occurs at the point of attachment of the hypostoma. 

 It is a very common occurrence to find the "doublure" on the reflected 

 under margin lying free from the other parts of the head, in the shale, 

 and with the hypostoma attached. This fact leads to the conclusion 

 that a suture may pass around nearer the frontal margin in the same 

 manner as Holm describes it in 0. Tcjerulfl.* 



Glabella clavate, narrow at the base, reaching its gicatest width just 

 back of the anterior termination of the eye lobes ; it narrows rapidly 

 towards the rather sharply rounded frontal margin. Three pairs of 

 glabellar furrows occur as shallow depressions, the anterior one oppo- 

 site the point where the eye lobe merges into the frontal lobe of the 

 glabella ; the furrows on the opposite side extend in, but do not unite. 

 Occipital furrow shallow and extending back from each side towards 

 the center. Occipital ring narrow at sides and increasing rapidly in 

 width to the center, where it supports a long, strong spine that curves 

 back over the thorax ; none of the specimens show the entire spine, but 

 I think it extends back in the adult fully one-half the length of the 

 thorax. Eye lobes crescentiform, narrow, elongate, arching from the 

 base of the anterior lobe of the glabella, into which they merge, back 

 to a line with the occipital furrow and some distance from the glabella ; 

 visual surface unknown. The area between the glabella and eye lobe 

 is slightly depressed, a narrow, shallow furrow extending along the inner 

 edge of the eye lobe. The frontal limb and cheeks slope gently to the 

 ridge within the outer margin. No traces of facial sutures observed, 

 although on some of the casts of the inner side of the shell a depressed 

 line in the shell is indicated by a raised line on the cast. This line fol- 

 lows the direction I should theoretically give to the suture. Hypostoma 

 moderately convex, broad in front and narrowing towards the posterior 

 margin.t One specimen is 18 mm across the greatest width, and li""" 

 across the posterior end. The anterior margin shows a rounded, smooth 

 edge that fits into the slight, curved recess of the "doublure" of the 

 head except laterally, where it extends out to meet the side margin of 

 the anterior wings to form a blunt point; back of the anterior wings 

 the margin is raised to form an elevated rim and then curves uuder ; 

 the rim extends around to and across the posterior margin, becoming 

 most prominent at the posterolateral angles; the marginal rim is sep- 

 arated from the body by a sulcus that disappears on the anterior wings ; 

 the posterior groove, iu front of the marginal sulcus, is well defined and 



"Aftryck vr. Geol. Foreu. i Stockholm. Forhandl., Bel. ix, Haft 7, 1887, p. 16. 



tThe front margin is the point of attachment to the head and the posterior mar- 

 gin, the margin next to the mouth of the animal and facing the posterior margin of 

 the head. 



