In two previous papers in this Magazine (Hawkins, 1910 and 1911) I have dealt with some features in the comparative anatomy of the ambulacral and interambulacral areas of the Holectypoida. The only other structure of the test for a comparative study of which sufficient material is available is the apical system. The present paper deals with this structure. I intend to summarize the results of my researches in a scheme for the classification and evolution of the group in a paper already partly prepared.
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Herbert L. Hawkins (1912) studied this question.