Abstract

The Devonian Nortonechinus [Lime Creek Shale, Iowa], a cidarid echinoid, and Lepidocentrus, an echinocystitid, have similar ambulacra with the radial vessel roofed over by perradial extensions of the ambulacral plates. The similarity of these two genera suggests that the cidarid stock probably descended from the echinocystitid stock. The following evolutionary trends apparently occurred within the cidarids: loss of the internal covering of the radial water vessel; reduction in the number of interambulacral columns; increase in the size of the interambulacral plates; decrease in imbrication of the plates; increase in size and complexity of the tubercles; increase in the size of the spines. It is suggested that most of these changes were related to the greater utilization of the primary spines.

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