Seal of the United States Customs Service, when it was part of the Department of the Treasury.  The service has since been reorganized and split up into two agencies under the Department of Homeland Security, mainly the U.S. Customs and Border Protection.  The design is the same as the Treasury seal with a Customs Service inscription.
Extracted from PDF version of Pets and Wildlife informational brochure (direct PDF URL [1]).
U.S. Government