(a) Exception. Section 670.4 shall not apply to:

(1) any native mammal, bird, or plant which is held in captivity on or before October 28, 1978; or

(2) Any offspring of such mammal, bird, or plant.

(b) Presumption. With respect to any prohibited act set forth in §670.4 which occurs after April 29, 1979, the Act creates a rebuttable presumption that the native mammal, native bird, or native plant involved in such act was not held in captivity on or before October 28, 1978, or was not an offspring referred to in paragraph (a) of this section.