738 THE PALEONTOLOGY OF MINNESOTA. 
{Note on Ceraurus. 
We may summarize and tabulate the subgeneric relations of the best known 
American representatives of Ceraurus, in the following manner: 
1. Pygidium with first pair of ribs much produced and embracing the short extension of the other 
one or two pairs. 
a. Glabella depressed, subquadrate; glabellar furrows short and horizontal: Ceraurus s. s. 
C. pleurexanthemus Green. i 
C. polydorus Billings. 
C. pompilius Billings. 
C. nuperus Billings. 
C. tarquinius Billings. 
b. Glabella ovoid, convex; glabellar lobes posteriorly oblique, third lobe not separated; ocular 
ridge continuous with frontal border: Cyrtometopus. 
C. apollo Billings. 
C. mercurius Billings. 
C. rarus Walcott. 
C. scofieldi Clarke. 
ce. Glabella elliptical, convex behind, with obliquely posterior furrows; third lobes not separa- 
ted; usually a spine on the posterior portion of the glabella: Nieszkowskia. 
C. glaucus Billings. 
C. perforator Billings. 
C. satyrus Billings. 
C. numitor Billings. 
d. Frontal and first and second lateral lobes confluent and highly convex or subspherical: 
Spherocoryphe. 
S. robusta Walcott. 
S. salteri Billings. 
2. Pygidium with free pleural ribs not extending beyond the posterior arc. 
e. Glabella subquadrate, rounded and full in front; lateral furrows short and horizontal; 
third lobe not separated; pygidium with four pairs of broad obtuse marginal 
extensions: Heccoptochile. 
C. icarus (Billings) Meek. 
f. Glabella with large and convex frontal lobe; members of first and second lateral furrows 
confluent; third lobes wholly isolated. Pygidium with six distant, sharp, 
incurved caudal spines: Crotalocephalus. 
C. niagarensis Hall (—C. insignis Beyrich.) 
g. Glabella convex, ovoid or subtriangular; frontal lobe small, lateral furrows oblique; third 
lobe large and not isolated. Pygidium with eight divergent spines: Pseudo- 
spheerexochus. 
C, prolificus Billings. 
P. trentonensis Clarke. 
