 In most Osteichthyes, the skull is only loosely joined, and the endocranial elements do not form a unit with the skull roof.An endocast or endocranial cast is a cast made of the mold formed by the impression the brain makes on the inside of the neurocranium (braincase), providing a replica of the brain with most of the details of its outer surface.Endocasts can also form naturally, when sediments fill the empty skull, after which the skull is destroyed and the cast fossilized.