PALAEONTOLOGY OF IOWA. 681 



COLUMN round, composed near the body of alternating 

 thicker and thinner joints. 



This species may be distinguished by its small size, the parallel sides of the third 

 radial plates, and the elongate first arm-joints. 

 Fig. 3 a. View of anterior side of specimen. 



Fig. 3 b. The same enlarged, showing more distinctly the form and arrangement of 

 the plates. 



FIG. 108. 



The accompanying diagram illustrates the structure 

 of this species, which, in the form and proportions of 

 the plates of the body and arms contrasts conspicuously 

 with the Preceding species. 



Geological position and locality. In the Kaskaskia limestone : Chester, 

 Illinois, 



Zeacrinus intermedius ( n. s.). 



PLATE XXV. FIG. 4. 



BODY broad, abruptly spreading to the base of the first 

 radials. Basal plates minute, hidden by the column. Subradial 

 plates longer than wide, hexagonal, except the anal and right 

 postero-lateral plates, which are heptagonal. First radial 

 plates pentagonal, short, nearly twice as wide as long. Second 

 radial plates short, pentagonal ; each one, on the upper 

 sloping sides, supporting two diverging arms composed of 

 short plates, which, with the exception of the lower ones, 

 are more than twice as wide as long. Arms bifurcating on 

 the third, fourth, fifth or sixth plate above the base, and the 

 outer divisions of each ray again bifurcating on the sixth 

 plate above the first division. Anal plates six determinable. 



COLUMN small, pentagonal, consisting of alternating longer 

 and shorter joints, or of several shorter ones between the 

 longer ones : the larger joints are thickened at their angles, 

 and bear slender rounded branching rootlets. 

 [lowA SURVEY.] 86 



