The Echinoid Directory

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Key to genera of Heterodiadematidae 1a. Apical disc prolonged posteriorly in adults so that the opening has a key-hole shape. Heterodiadema 1b. Apical disc not prolonged posteriorly in adults. Go to 2 2a. Primary ambulacral tubercles hardly differentiated so that ambulacral zone looks uniformly granulated. Monodiadema 2b. Primary ambulacral tubercles distinct at ambitus and adorally. Go to 3 3a. Primary tubercles large and prominent up to ambitus but small and marginal above the ambitus. Trochodiadema 3b. Primary tubercles small and marginal throughout. Ekerekeia