‖n. [ NL., from Gr. 'a priv. + &unr_; skull. ] 1. (Physiol.) Partial or total absence of the skull. [ 1913 Webster ]
2. pl. (Zool.) The lowest group of Vertebrata, including the amphioxus, in which no skull exists. [ 1913 Webster ]
a. Wanting a skull. [ 1913 Webster ]
a. Within the cranium or skull. Sir W. Hamilton. [ 1913 Webster ]
a. (Anat.) Situated above, or in the roof of, the cranium. [ 1913 Webster ]