TRILOBITA—OPISTHOPARIA. 



293 



Fig. 1 60 1. Isotelus gigas, 

 narrow form, reduced. (After 

 Hall.) 



Fig. 1602. Iso'elus gigas, broad Jorm. 

 (After Logan.) 



Fig. 1603. Iso^e/us'^l maximus. 

 (After Logan.) 



Fig. 1604. lUcenus americanus, 

 with the outline of the cheek of /. con- 

 similiSy dotted in a. (After Billings.) 



