562 



Notices of Memoirs- 



correlation was long a matter of uncertainty, until Mr. Nason 

 described certain horizons bearing an abundant and characteristic 

 Cambrian fauna. 



The specimen here described at once suggests the familiar and 

 well-known genus Eurypteriis, and only when its characters are 

 studied in connection with its geological occurrence is it apparent 

 that its differences are of sufficient importance to warrant its generic 

 separation. The specimen represents nearly the entire dorsal test 

 of the animal, and consists of the cephalothorax with the abdominal 

 segments, including the telson or tail-spine. 



Strabops Thacheri, Beecher. — Dorsal aspect of t}'pe- specimen, \. Potosi Limestone 

 (Cambrian) : St. Francois County, Missouri. Original in Tale University 

 Museum. 



The cephalothorax is comparatively shorter and wider than in 

 EuryTpterus, the eyes are further forward, nearer together, and more 

 oblique, and besides the telson but eleven abdominal somites can 

 be determined on the dorsal side, instead of twelve as in Eurypterus. 



