You are joking - right?
Sharks are not mammals, they are a species of fish called Elasmobranchs, not a boney fish as they have no bones, but have cartilage instead.
http://boards.straightdope.com/sdmb/showthread.php?t=327069
“Well, “fish” is not really a technical scientific term.”
“According to Merriam Webster, a fish is:”
“Quote:”
“any of numerous cold-blooded strictly aquatic craniate vertebrates that include the bony fishes and usually the cartilaginous and jawless fishes and that have typically an elongated somewhat spindle-shaped body terminating in a broad caudal fin, limbs in the form of fins when present at all, and a 2-chambered heart by which blood is sent through thoracic gills to be oxygenated”
“”Fish” thus normally includes three different living classes of vertebrates: the Osteoichthyes, or bony fish; the Chondrichthyes, or cartilaginous fish (sharks and rays, as well as some odder groups such as chimeras), and the Agnata, or jawless fish (lampreys, hagfish, plus many early fossil forms). There is yet another class, the Placoderma, which includes armored fish that are all now extinct.”
“So yes, sharks are indeed fish, but a quite different kind of fish than a trout. The two are about as closely related as an elephant is to a turtle.”
So, you win.