736 THE PALEONTOLOGY OF MINNESOTA. 



[Note on Ceraurus. 



pairs of glabellar furrows are exceedingly obscure and may be discerned only in an 

 oblique light. Occipital furrow narrow and deeply impressed; occipital ring broad 

 in the middle, highly arched on the axis, which is scarcely as wide as the base of 

 the glabella. On the cheeks the occipital ring gradually widens, at its extremity 

 meets the outer marginal rim of the border, and is continued into a short, outwardly 

 directed spine. The occipital furrow is also sharply defined upon the cheeks. 



Eyes of moderate size, not elevated to the hight of the glabella. Suture normal ; 

 ocular ridge from the anterior angle of the eye to the frontal margin broad and 

 conspicuous. The entire surface of the shield is minutely and uniformly pustulose 

 over all its parts. 



This species has somewhat the general aspect of Ceraurus pleurexanthemus, 

 but differs from it characteristically in the obscure lobation of the glabella, the 

 surface granulation, and the shortness of the genal spines. It is, I believe, wholly 

 distinct from any of the various species " Cheirurus" described by Billings from the 

 Lower Silurian, though the usual imperfection of Billings' material renders a decisive 

 opinion impossible. 



Formation and locality. Lower Trenton limestone, with Ortiiis pectinella and Strophomenn subtenta, 

 Minneapolis; and the Glades, Lebanon, Tennessee. Collection of Mr. Ulrich. 



NOTE ON THE SUBGE-NERIC CLASSIFICATION OF THE AMERICAN SPKOIKS OF THE GENUS 



CERAUHI s. 



The wide variation in the form and degree of lobation of the glabella and form 

 of the pygidium in species which have been referred to Ceraurus, led Angelin, 

 Schmidt, and some others, to introduce a number of subordinate and useful 

 designations for what appear to be natural groups. The structure of Ceraurus as 

 exemplified by its type species, C. pleurexanthemus, is characterized by its subquad- 

 rate glabella with moderately large frontal lobe, short, subequal lateral lobes 

 .separated by horizontal furrows, the third lobes being apparently isolated by a 

 linear depression extending from the actual inner termination of the furrow to the 

 occipital groove. The surface of the inner cheeks is characteristically marked by 

 deep pittings scattered among the tubercles, while the pygidium bears two or three 

 pairs of marginal spines, the first being of very great length. 



The principal points of deviation from this type are manifested in the compo- 

 sition of the glabella. The glabella in Cyrtometopus, Angelin, Pseudosphaerexoc//ns, 

 and \ics:/,-tnrs/.-i>i, Schmidt, is subtriangular, broadly clavate or subovoid, the lateral 

 furrows and lobes being directed posteriorly. In Cyrtometopus the glabella is evenly 

 and not greatly convex, and the third lobe is usually not wholly separated from the 

 The ridge extending from the eyes forward to the anterior margin of the 



