Why are eels not kosher even though they have fins and scales?

While eels do have scales from a scientific point of view, the Word kashkeshet (קשקשת) is interpreted in the Talmud to be a subset of scales that can be cleanly removed from the skin of the fish. There are several types of scientifically-accepted scales which do not have this attribute, such as the denticles of a shark and the thin scales of an eel. It is for this reason that eel is not kosher.