39 U.S.C. § 3015 - Nonmailable plant pests and injurious animals

(a) Injurious Animals.— 
Any injurious animal, the importation or interstate shipment of which is prohibited pursuant to section 42 of title 18, constitutes nonmailable matter.

(b) Plant Pests.— 
Any plant pest, the movement of which is prohibited pursuant to section 103 or 104 of the Federal Plant Pest Act (7 U.S.C. 150bb or 150cc),^[1] constitutes nonmailable matter.

(c) Plants.— 
Any plant, article, or matter, the importation or interstate shipment of which is prohibited pursuant to the Act of August 20, 1912 (37 Stat. 315, chapter 308; 7 U.S.C. 151 et seq.) (commonly known as the Plant Quarantine Act),^[1] constitutes nonmailable matter.

(d) Illegally Taken Fish, Wildlife, or Plants.— 
Any fish, wildlife, or plant, the conveyance of which is prohibited pursuant to section 3 of the Lacey Act Amendments of 1981 (16 U.S.C. 3372), constitutes nonmailable matter.