Alive at the crossroads between Elliptical and Spiral galaxies in Hubble's tuning-fork diagram are the S0 class of galaxies. Seen below is NEMUG2009-X:
Author's note: NEMUG = Non-exsistent Made-up Galaxy Seen in the above example are two complex and very energetic jets streaming away from the galaxy's nucleus as well as its dark dust lane and sister galaxy at lower left.
While much is known about the evolution of stars the opposite is true for galaxies. While more recent scientific thought and research on the evolution of galaxies is based in the study and understanding of their stars and stellar populations, Hubble's visual classification scheme remains the best indicator of a galaxy's structural evolution and not a definitive diagram or tool for evolutionary developement, a process that remains one of the unresolved challenges of astronomy today. from The Galaxies Within Our Universe, Spirals - Hunting for The Perfect Galaxy K. Pinkela