204 SMITHSONIAN INSTITUTION 



This species, the more elongate form of the genus, belongs to the Upper Burlington lime- 

 stone. One imperfect specimen, considerably larger than any others but otherwise scarcely 

 distinguishable from this, has been found in the lower passage beds of the Keokuk limestone. 

 In one specimen (fig. 2), exceptionally, the primibrachs are increased from their normal 

 number to four. The succeeding - intervals between the bifurcations are very long, and this 

 constitutes the best distinction from the Lower Burlington species ; only in one specimen, ap- 

 parently intermediate in character, do they fall as low as six brachials, those of this order 

 being usually eight or nine, and the IIIBr rarely less than 14. 



Types. The original of figure 1 is in the Museum of Comparative Zoology, Harvard 

 College. The other figured specimens are in the author's collection. 



Horizon and locality. Lower Carboniferous, Upper Burlington limestone; Burlington, 

 Iowa. 



