TRILOBITA—OPISTHOPARIA. 



291 



smooth. Free cheeks large. Eyes large and prominent. Hypo- 

 stoma deeply forked posteriorly. Thorax of eight segments. 

 Pleura grooved, with rounded extremities. Axis rather narrow. 

 Pygidium trilobed, its axis distinctly segmented, the side lobes 

 slightly segmented. Ordovicic. 



90. A. marginalis Hall. (Fig. 1599.) Ordovicic. 



Fig. 1599. Asaphus viarginalis : a, 

 small cephalon ; b, free cheek ; r, pygidium. 

 (After Raymond.) 



Fig. 1600. hotelus canalis, free 

 cheek, cranidium and pygidium, all X 

 yi. (After Whitfield.) 



Cephalon wide in front of glabella. Fixed cheeks very narrow. 

 Segmentation of pygidium not reaching the border, leaving a 

 smooth margin. 



Beekmantown (?) of Pennsylvania; Chazy of New York. 



XLI. IsoTELUS Dekay. 

 Differs from Asaphus in its broad axis and obsolete segmenta- 

 tion, at maturity, of glabella and pygidium. Ordovicic. 



91. I. canalis Conrad. (Fig. 1600.) Ordovicic. 

 Cephalon with nearly rectangular, faintly defined glabella and 



moderate genal spines; pygidium with comparatively narrow and 

 well defined axis. 



Beekmantownian of New York, Vermont, Canada, etc. 



92. I. obtusus Hall. Ordovicic. 

 Differs from /. gigas in its relatively broader glabella, thoracic 



