An enclosing structure or sheath, or a part of one, which is primary in nature or basal in position; especially (a) (in a coral polyp) the theca in its initial or rudimentary form (rare); (b) (in a foraminiferan of the superfamily Fusulinacea) an unmodified primary wall; (c) (in a graptolite of the order Graptoloidea) the basal part of a theca, attached to the sicula.