Absence of the skull

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Acrania \A*cra"ni*a\, n. [NL., from Gr. 'a priv. + ? skull.] (Physiol.) Partial or total absence of the skull. pl. (Zo["o]l.) The lowest group of Vertebrata, including the amphioxus, in which no skull exists.

Wiktionary Word definitions in Wiktionary
n. (context physiology English) Partial or total absence of the skull.

Wikipedia Word definitions in Wikipedia
Acrania is a rare congenital disorder that occurs in the human fetus in which the flat bones in the cranial vault are either completely or partially absent. The cerebral hemispheres develop completely but abnormally. The condition is frequently, though ...