Whereas some modern whales possess a small and useless pelvis, linking modern whales to their four-legged evolutionary ancestors possess a larger and even functional pelvis.

VESTIGIAL; TRANSITIONAL FORM

Explanation:Vestigial organs are organs whose functions are not known,many organism have Vestigial organs. In Whales it's Pelvis bone is called a Vestigial organ.

Transitional Form is a body structure through which whales or other organisms can be related or be known to have evolved from their ancestry.

The ancestors of Whale have four leggs with more functional Pelvis. Modern Whales are believed to have evolved or transited from

Ambulocetus (Kutchicetus, etc).