TRILOBITA—PROPARIA. 319 



LXIII. Ceraurus Green. 



Glabella strongly convex, subquadrate or expanding in front, 

 its width one third less than that of the cephalon, rounded and 

 prominent anteriorly, and with three lateral furrows on each side. 

 Eyes small, minutely facetted, considerably removed from the 

 glabella. Genal spines on the fixed cheeks. Facial sutures shown 

 in Fig. 1632. Thorax usually with 11 segments, rarely with 9 

 to 13. Pleura flattened for a distance and then curved downward 

 and backward. Pygidium small, with segments terminating in 

 projections. Ordovicic and Siluric. 



The subgenus Crotalocephalus Salter has the posterior gla- 

 bellar lobes wholly isolated; the pygidium with the free pleural 

 segments ending in six distant, sharp, incurved spines. (Type 

 C. niagarensis.) 



162. C. (Crotalocephalus) hudsoni Raymond. Ordovicic. 

 Differs from C. pompilius in having the glabella expanded toward 



the front and in the more prominent pustules of the surface. 

 From C. pleure.ranthemus it differs in having the cheeks smaller 

 and more convex, the eye farther forward and the posterior gla- 

 bellar lobes isolated. 

 Chazy of New York. 



163. C. (Crotalocephalus) pompilius Billings. Ordovicic. 

 Glabella subrectangular ; posterior glabellar lobe subquadrate. 



Surface papillose. 



Chazy of New York, Quebec, etc. 



164. C. pleurexanthemus Green. (Fig. 1632.) Ordovicic. 

 Glabella broadest in front. Whole surface of head strongly 



papillose. (Type of genus.) 



Lowville-Lorraine (especially Trenton) throughout most of 

 North America. 



165. C. (Crotalocephalus) niagarensis Hall. (Fig. 1633.) Siluric. 

 Neck furrow arches forward and joins the posterior glabellar 



furrow, thus making the posterior glabellar lobe triangular. The 

 convex lateral and posterior borders are, at their junction, pro- 

 duced straight backward into a slender genal spine. 



Niagaran of New York, Indiana, Tennessee, Wisconsin, etc. 



