IOWA ACADEMY OF SCIENCES. 79 



VARIATION IN THE POSITION OF THE NODES ON 



THE AXIAL SEGMENTS OF PYGIDIUM OF 



A SPECIES OF ENCRINURUS. 



BY WILLIAM HARMON NORTON. 



In defining the different species of the genus encrinurus 

 (Emmrich) use has frequently been made of the disposition of 

 nodes on the rings of the mid-lobe of the tail- shield. It is 

 largely by this diagnostic that Foerste, for example, distin- 

 guishes E. thresheri from E. ornatus. Hall and Whitfield* 

 and the latter authors again, use the same criterion in separat- 

 ing E. oimatus from the European species figured in Murchi- 

 son's Siluria. f 



This has been the perhaps unavoidable result of the scarcity 

 of materials at hand. Several species of this genus have been 

 described, each from a single pygidium. The specific impor- 

 tance of this feature having thus been exaggerated, any varia- 

 tion in it is of paleontological as well as evolutional interest, 

 and will be of value in the long-needed revision of the genus. 



The specimens which afford the facts I am about to present 

 were taken some years since by Prof. A. Collins, Sc.P , of Cor- 

 nell College, and the author, from a single stratum near the top 

 of Platner & Kirby's quarry, Mount Vernon, Iowa. They were 

 associated with a rich fauna, but unfortunately the fossilif erous 

 area was so limited that, though the quarry has been largely 

 extended, scarcely a fossil has since rewarded the search of the 

 collector. The investigation is therefore simplified by the 

 absence of such factors as would obtain if the specimens had 

 been taken from widely separated localities, or from a consid- 

 erable vertical range. 



Coming from a station near the summit of the Anamosa beds, 

 which lie above the Le Claire, the position of the species is 

 perhaps higher than that of any other American Eacrinurus. 



* The Clinton Group of Ohio, Part II, pp. 101, 103, A. E. Foerste. Bulletin of The 

 Xiaboratories of Denison University, II. 



+ Eeport Geological Survey of Ohio. Vol. II, pp. 155, 156. 



