Fell, Howard B. Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts.
- Additional Readings
A subclass of Echinoidea lacking stability in the number of columns of plates that make up the ambulacra and interambulacra. The ambulacral columns vary from 2 to 20, the interambulacral from 1 to 14. There are two orders, Bothriocidaroida and Echinocystitoida. See also: Echinodermata; Echinoidea
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