Cystoids (sis'-toids) are related to the crinoids and blastoids but are more primitive than either. The body, or calyx, is not nearly so well developed, and the arms are irregular and rarely preserved. The body plates are quite irregular in arrangement.
Many cystoids found in Illinois come from quarries in Silurian rocks of the Chicago region and in the Mississippi River bluffs of northwestern Illinois.
See cystoids: plate 17 (bottom half of image).
The printed version of Guide for Beginning Fossil Hunters can be purchased from the Shop ISGS Web site.
Updated 09/23/2011 SLD