(a) General. (1) Within 1 year of the publication of a rule proposing to determine whether a species is an endangered or threatened species, or to designate or revise critical habitat, the Secretary shall publish one of the following in the Federal Register:

(i) A final rule to implement such determination or revision,

(ii) A finding that such revision should not be made,

(iii) A notice withdrawing the proposed rule upon a finding that available evidence does not justify the action proposed by the rule, or

(iv) A notice extending such 1-year period by an additional period of not more than 6 months because there is substantial disagreement among scientists knowledgeable about the species concerned regarding the sufficiency or accuracy of the available data relevant to the determination or revision concerned.

(2) If an extension is made under paragraph (a)(1)(iv) of this section, the Secretary shall, within the extended period, take one of the actions described in paragraphs (a)(1) (i), (ii), or (iii) of this section.

(3) If a proposed rule is withdrawn under paragraph (a)(1)(iii) of this section, the notice of withdrawal shall set forth the basis upon which the proposed rule has been found not to be supported by available evidence. The Secretary shall not again propose a rule withdrawn under such provision except on the basis of sufficient new information that warrants a reproposal.

(b) Critical habitat designations. A final rule designating critical habitat of an endangered or a threatened species shall to the extent permissible under § 424.12 be published concurrently with the final rule listing such species, unless the Secretary deems that—

(1) It is essential to the conservation of such species that it be listed promptly; or

(2) Critical habitat of such species is not then determinable,

in which case, the Secretary, with respect to the proposed regulation to designate such habitat, may extend the 1-year period specified in paragraph (a) of this section by not more than one additional year. Not later than the close of such additional year the Secretary must publish a final regulation, based on such data as may be available at that time, designating, to the maximum extent prudent, such habitat.