Thanks for looking up the definition, Lynn. I would respectfully disagree with this take on the definition, however. Transitional fossils/forms provide links both with regard to morphology and with respect to time. This is what is meant by ancestors and progeny, I would assert.
Therefore modern sharks should not be regarded as transitional forms, as they are not intermediate in time between ancestors and progeny. On the other hand, Xenacanthus is an excellent example of a transitional fossil leading to early sharks.
Best,
Chris