PALAEONTOLOGY OF IOWA. 549 



single series, having the edges thicker on one side than on 

 the other. Tentacula originating from the thicker sides of 

 the arm-plates, tapering somewhat abruptly from the base, 

 and becoming slender and numerously jointed. 



SURFACE finely and beautifully granulate in fine undulating 

 and somewhat radiating striae. 



This pretty species is clearly congeneric with the preceding Z. elegans. It is a more 

 slender species, with some differences in the basal and radial plates; while the bi- 

 furcations of the arms offer more palpable means of distinction in the number of 

 plates between each bifurcation. The plates of the arms are likewise proportionally 

 longer. 



Fig. 3. Antero-lateral side of specimen, showing the radial plates and the anterior ray 

 on the left side of figure, with arras and tentacula. On that side two of the 

 subradial plates are partially broken off, so that the figure does not give the 

 form of these plates entirely. 



FIG. 69. 



The accompanying figure illustrated the structure of the 

 body and lower part of the arms of this species, the ante- 

 rior ray showing the irregularity already noticed. The base 

 is too small to admit of marking the lines of subdivision. 



Geological formation and locality. In the Burlington limestone : Bur- 

 lington, Iowa. 



Genus POTERIOCRINUS : Submenus Scaphiocriiius*. 



Associated with the ZEACRINUS there is another group of species, which differ in 

 general aspect sufficiently to be readily recognized, but which, on strict comparison, 

 do not differ in their essential structure from that genus. Like that, they begin their 

 existence in the Burlington limestone period, where the first noticed form has the 

 aspect and character of GRAPHIOCRINUS. 



This species is figured on Plate ix, fig. 10; and, as shown under the description, 

 possesses one series of plates more than GRAPHIOCRINUS, having five subradials above 

 the five basal plates ; and while the arms in this species are not bifurcated above their 

 origin on the second radial, in another species, having the same structure of body, 

 the arms dichotomize at a point about half way from the base to the summit. Farther 

 comparison has shown other species having the arms more or less bifurcating, but 

 preserving generally a slender elongate form. The base is not concave as in ZEA- 

 CRINUS, and the five small basal plates are often quite conspicuous in a lateral view. 



* For generic descriptions and illustrations of the Genera CYATHOCKINUS and POTERIO- 

 CRINUS, see under Crinoideae of the Keokuk limestone. 



