TKiLOBirKs OF THR Chazv Limestonk. 375 



eye, the relative size of the fixed and free cheeks, and the ])roportion 

 of the glabella to the rest of the ce[)halon, we arrive at the following 

 grouping for the American Ordovician forms. This, it will be seen, 

 differs from Clarke's only in the grouping of the species under two 

 of the subgenera, although the subgenera must be redefined. The 

 subgenus, A^icszkoti'skia, might be useful to designate a senile stage of 

 the genus Psendosf^licerexochus, but it can hardly be separated, as there 

 are within the range of variation of one species, Cheiniriis vtilcauiis, 

 all gradations from typical FseiidospJuerexochits to typical Nicszkowskia. 



1. Ceraiinis sensu stricto. 



Free cheeks eciual to or smaller than fixed cheeks. Eyes consider- 

 ably removed from glabella. Glabella, usually subquadrate or ex- 

 panding in front, less than one third the cephalon. Genal angles 

 bear spines. Three pairs of glabellar furrows. 



Ceraiinis pleurexantJicimts Careen. 



C. Jmdsoni Raymond. 



C. pompilius Billings. 



C. niipenis Billings. 



C. tarquinus Billings. 



C. scofieldi Clarke. 



2. Sp/uerocofphc. 



Free cheeks smaller than fixed cheeks. Eyes far from glabella. 

 Glabella a larger proportion of cephalon than in Ceraurus. Frontal, 

 first and second lobes of glabella confluent. Genal angles bear spines. 



Splurrocorphe rohusfa Walcott. 

 S. salteri Billings. 

 6". major Ruedernian. 

 5. goodnoi'i Raymond. 



3 . Fseudosp/icerexochus. 



Free cheeks larger than fixed cheeks. Eyes very close to the gla- 

 bella. Glabella one third or more the width of the cephalon. Genal 

 angles generally rounded. 



In the section Nieszkowskia the glabella is very convex, abrupt be- 

 hind, or produced into a spine. 



PseudosphcErexochiis ireutonensis Clarke. 



P. chazyensis Raymond. 



P. approximus Raymond. 



