Article: Tooth structure of the pygasteroid sea urchin Plesiechinus

The fine structure of a tooth thought to belong to the Jurassic pygasteroid Plesiechinus ornatus (Buckman) is described and the tooth elements are reconstructed. The tooth is quite distinct from contemporary and Recent keeled teeth of regular echinoids but has a structure similar to that of known teeth of irregular echinoids. The discovery that pygasteroids have teeth constructed like other irregular echinoid teeth so far described strengthens the view that irregular echinoids are a monophyletic group.