TRILOBITA—OPISTHOPARIA. 295 



98. I. latiaxiatus Raymond and Narraway. Ordovicic. 

 Very similar to /. americanus but cephalon is proportionally 



wider and has shorter, straighter and shallower dorsal furrows. 

 It differs especially in that the pygidium is much more strongly 

 truncated at the sides (almost at right angles to the anterior mar- 

 gin), less arcuate posteriorly, and with a more convex and promi- 

 nent axial lobe. The pleural lobes of the thorax are flat for about 

 half their width and are then abruptly deflected almost at a right 

 angle. 



Black River of New York and Ontario. 



99. I. americanus Billings. (Fig. 1604.) Ordovicic. 

 Strongly convex. Length of entire individual about one and 



one fourth inches; greatest width of body at posterior portion of 

 cephalon one inch. Transverse fold present at anterior part of 

 cephalon. Eyes situated as in Bumastus trentonensis. Triloba- 

 tion of head confined to posterior part, but here the glabella is quite 

 convex. Axis extends about half across the pygidium. Surface 

 of test marked with wave-like wrinkles. Thoracic axis slightly 

 broader than in /. crassicauda, an axis three fourths inch in length 

 being over one half inch in width; pygidium also proportionally 

 smaller. 



Trenton of New York and Ontario ; Galena of Illinois, Wiscon- 

 sin and Minnesota. 



100. I. crassicauda (Wahlenberg). Ordovicic. 

 Oval, convex. Trilobation extending a short distance into ceph- 

 alon, Cephalon large, with a short, raised rim, bordering its an- 

 terior margin; posterior extremities obscure. Pygidium semicir- 

 cular, very convex posteriorly. Entire surface smooth. Thoracic 

 axis of one specimen measured three fourths inch in length by less 

 than one half inch in width. (Type of genus.) 



Trenton of Vermont, New York, Pennsylvania, Nevada, etc. 

 loi. I. ioxus Hall. (Figs. 1606, 1607.) Siluric. 



Large. Cephalon subtruncate in front; dorsal furrows broad 

 but not deeply impressed, obsolete in front of the eyes. Eyes large, 

 narrow, near the posterolateral border of the cephalon. Free 

 cheeks wide, bounded inwardly by a narrow ridge parallel with and 

 directly beneath the eye ; this ridge marks the inner margin of the 

 depressed area of the cheeks. 



