Some galaxies cannot be classified as elliptical or spiral galaxies. They are too misshapen to find a place in the Hubble diagram.
They are called irregular galaxies ; here are two of them.
The Large Magellanic Cloud, the nearest galaxy.
Source : AURA/ NOAO/ NSF
M82, an irregular galaxy in the Big Dipper, as seen by the Subaru telescope.
Source : NOAJ.
These galaxies are often small, between 10 million and one billion stars. They frequently have a lot of gas, so they are active regions of star formation, just like the disks of the spiral galaxies.
They are also quite dim, and only the nearest of these galaxies can be easily detected.
They may have been very numerous in the ancient universe, but their number has decreased, due to collisions. The remaining ones have now a low brightness which prevents them being detected.
To finish this classification, the diagram below summarizes the main differences between a spiral galaxy and an elliptical one.
A full page of galaxy images to look at.
You will find spiral galaxies, irregular ones, and colliding galaxies.