Class Mammalia (mammals)
Subclass Theria (Therian mammals)
Infraclass Eutheria (placental mammals)
Superorder Cetartiodactyla -- containing:
Order Cetacea (dolphins, porpoises, and whales)
Order Artiodactyla (even-toed ungulates*)
Cetartiodactyla is a superorder proposed to denote a clade including the orders Cetacea and Artiodactyla.
There are two different interpretations of the relationship between Cetacea and Artiodactyla in the superorder Cetartiodactyla. They may be sister groups, both evolved from a common ancestor, one of the extinct Mesonychia. Alternatively, Cetartiodactyla may refer to a classification in which the Cetacea actually evolved from within the Artiodactyla.
If the whales did evolve from the Artiodactyla, then the hippopotamus would be the whale's closest living land mammal. The clade that unites the hippo and the whales is called Cetancodonta.
The term Cetartiodactyla was created by merging the names of the two orders, Cetacea and Artiodactyla, into a single word.
* Okay, you don't have to go to the ungulates node. The
even-toed ungulates include pigs, hippos,
peccaries, camels, llamas, deer, giraffe, cattle, goats, sheep, and antelope, among others.
References:
http://animaldiversity.ummz.umich.edu/site/accounts/classification/path/Cetartiodactyla.html#Cetartiodactyla
http://en.wikipedia.org/wiki/Cetartiodactyla
http://www.answers.com/topic/cetartiodactyla