One well-known characteristic that separates the class of mammals from other animals is the mammary glands.
(One well-known characteristic; separates; the class of mammals from other animals) (One well-known characteristic that separates the class of mammals from other animals; is; the mammary glands)
One well-known characteristic One well-known characteristic that separates the class of mammals from other animals
separates is
the class of mammals from other animals
the mammary glands