The radix entomolaris is an additional root in human's mandibular molar teeth. The human mandibular teeth have two roots usually. In rare cases, however, a root may develop between the distal and the mesial roots which is called entomolaris, if it is located lingual to the tooth. Such a phenomenon is named radix paramolaris in case an extra root is buccal to a molar tooth. It is important to remember that an extra root can appear both in the first and second molars.
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Radix entomolaris Wikipedia(Text) CC BY-SA