Merriam-Webster's Word of the Day for July 31, 2010 is:

sirenian • \sye-REE-nee-un\  • noun
: any of an order (Sirenia) of aquatic herbivorous mammals (as a manatee, dugong, or Steller's sea cow) that have large forelimbs resembling paddles, no hind limbs, and a flattened tail resembling a fin

Example sentence:
"Looking humanlike in certain aspects, sirenians are thought to be the basis of the myth of mermaids." (Michael McCarthy, The Independent [London], February 28, 2009)

Did you know?
"Sirenian" traces back via Latin to Greek "seirēn," which is equivalent to our word for the sirens of Greek mythology. And what is the connection between sirens and sirenians? Modern sirenians do not resemble the half bird, half woman creatures who lured sailors to their doom with their sweet singing. But as our example sentence states, sirenians are considered by some to underlie the ancient legends about mermaids. In European folklore mermaids were sometimes called "sirens," and apparently this confusion resulted in the granting of sirenians the name they bear today.