That would be a "joint work", which is not in the list. What is in the list is "a contribution to a collective work", which has to be distinct from the whole.
http://www.copyright.gov/title17/92chap1.html#101 : <<A “collective work” is a work, such as a periodical issue, anthology, or encyclopedia, in which a number of contributions, constituting separate and independent works in themselves, are assembled into a collective whole.>> <<A “joint work” is a work prepared by two or more authors with the intention that their contributions be merged into inseparable or interdependent parts of a unitary whole.>>
http://www.copyright.gov/title17/92chap1.html#101 : <<A “collective work” is a work, such as a periodical issue, anthology, or encyclopedia, in which a number of contributions, constituting separate and independent works in themselves, are assembled into a collective whole.>> <<A “joint work” is a work prepared by two or more authors with the intention that their contributions be merged into inseparable or interdependent parts of a unitary whole.>>