Science Dictionary
synapsid  [%PREMIUM_LINK%]     (sĭ-nāp'sĭd)  Pronunciation Key 
A reptile with one temporal opening on each side of the skull. Synapsids evolved in the late Permian Period and were characterized by carrying their limbs under their body and developing front teeth that were different from their back teeth. One group of synapsids, the therapsids, gave rise to the mammals. Compare anapsid, diapsid, therapsid.

The American Heritage® Science Dictionary
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