III.—On the Structure and Evolution of the Phyllodes in some Fossil Echinoidea

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Abstract
In all the Regular Echinoids, with the exception of the Cidarids, and in the Irregular Echinoids other than the Clypeastroids and Spatangids, there is a tendency for the ambulacral plates to become crowded together as they approach the margin of the peristome. This is the inevitable result of the continuous formation of fresh plates at the edges of the oculars in the apical system, and their less rapid resorption at the opposite extremity of the ambulacrum. The compression of the ambulaeral plates results in a crowding of the pore-pairs in the peristomial region, and this character seems to serve a sensory as well as a motor purpose.

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