Bone that is bilaterally paired endochondral bone with extensions of membrane bone forming part of a dorsolateral protective wall around the neural canal between the back of the skull and the beginning of the neural arch series. The first Weberian ossicle. Most recently, de Pinna and Grande (2003) have proposed the claustrum is homologous to the accessory neural arch of lower teleosts. [ ORCiD:0000-0001-9114-8737 GOC:mh ]