IOWA ACADEMY OP SCIENCES. 
79 
VARIATION IN THE POSITION OP THE NODES ON 
THE AXIAL SEGMENTS OP PYGIDIUM OP 
A SPECIES OP 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 Poerste, for example, distin- 
guishes E. threslieri from E. ornatus. Hall and Whitfield* 
and the latter authors again, use the same criterion in separat- 
ing E. ornatus 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 fossiliferous 
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 Encrinurus. 
* The Clinton Group of Ohio, Part II, pp. 101, 102, A. E. Foerste. Bulletin of The 
-Laboratories of Denison University, II. 
+ Report Geological Survey of Ohio. Vol. II, pp. 155, 156. 
