Fossil shells of the gastropods Hystrivasum locklini and Hystrivasum horridum from the upper Pliocene Pinecrest Beds (2.0 to 3.5 million years old), Sarasota, Florida. These species are among many that became extinct during the late Pliocene only to be replaced by new species, yielding a biota whose diversity has not changed in 3.5 million years. See page 1626. Other evolutionary events related to the closing of the Isthmus of Panama are discussed on pages 1603, 1624, and 1629. [Photo: Joe Traver; specimens are from the Paleontological Research Institution, Ithaca, NY]

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